Q:

Squares for answer choice options and perfect alignment to mathematical answers

Since 2005, I have generally used this model to create multiple-choice tests for my students. I kindly ask you if there are packages or macros, or particular strategies to have squares with the letters A, B, C, D inside. I created the letter E with the underline (tabular) if, for pure typing error or distraction, I had not put the correct answer. 
As you can see with the red arrows I do not have perfect squares and I think that all this depends on the letter used. But is it possible to find a solution to not use tabular? There is also another problem. Should I use aligned or other strategies to get a perfect alignment in mathematical mode? In which way could I have an optimal result? 

Here there is my MWE:
\documentclass[leqno,11pt,landscape,a3paper]{extarticle}
\usepackage{graphicx,multicol,mathtools}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\begin{document}

\begin{multicols}{1}
\begin{enumerate}

\item La velocità di 1,00$\cdot 10^2$ m/s espressa in km/h è:

$\fbox{A}$  36 km/h.\\
$\fbox{B}$    360 km/h.\\
$\fbox{C}$  27,8 km/h.\\
$\fbox{D}$    $3,\!60\cdot 10^{8}$ km/h.\\
$\fbox{E}$ \begin{tabular}{lllllllllllll}
\multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{6}{c}{} \\
\hline
\end{tabular}

\item In fisica nucleare si usa l'angstrom (simbolo: 1\AA=$10^{-10}$ m) e il fermi o femtometro (1 fm= $10^{-15}$ m). Qual è la relazione tra queste due unità di misura?

$\fbox{A}$  1 \AA = $10^5$ fm.\\
$\fbox{B}$    1 \AA = $10^{-5}$ fm.\\
$\fbox{C}$    1 \AA = $10^{-15}$ fm.\\
$\fbox{D}$    1 \AA = $10^{3}$ fm.\\
$\fbox{E}$ \begin{tabular}{lllllllllllll}
\multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{6}{c}{} \\
\hline
\end{tabular}

\end{enumerate}
\end{multicols}

\clearpage
\end{document}

A:

The obvious problem is that D is wider than A.
We can get a true square around capital letters by using the height as width:
\documentclass{article}

\newcommand{\squarecap}[1]{\fbox{\makebox[\height]{#1}}}

\begin{document}

\squarecap{A}\par
\squarecap{B}\par
\squarecap{C}\par
\squarecap{D}\par
\squarecap{E}\par
\squarecap{M}\par

\end{document}

The question about the angstrom is of a different kind: \AA=$1$ has no space; if you don't want to use siunitx (which is however recommended), type in
$1\,\textup{\AA} = 10^5\,\textup{fm}$

A:

You can use \eqmakebox[<tag>][<align>]{<stuff>} to have all <stuff> with the same <tag> be set <align>ed in a box of maximum width. Below I've played around with those so you can see what the effect is:

\documentclass{extarticle}

\usepackage[UTF8]{inputenc}

\usepackage{siunitx,eqparbox}

% https://tex.stackexchange.com/a/34412/5764
\makeatletter
\NewDocumentCommand{\eqmathbox}{o O{c} m}{%
  \IfValueTF{#1}
    {\def\eqmathbox@##1##2{\eqmakebox[#1][#2]{$##1##2$}}}
    {\def\eqmathbox@##1##2{\eqmakebox{$##1##2$}}}
  \mathpalette\eqmathbox@{#3}
}
\makeatother

\sisetup{
  per-mode = symbol
}

\newcommand{\makeoption}[2][default]{\fbox{\eqmakebox[#1]{#2}}}
\newcommand{\genericoption}{\rule[-.2\baselineskip]{100pt}{.4pt}}

\begin{document}

\begin{enumerate}
  \item La velocità di \SI{1e2}{\metre\per\second} espressa in \si{\kilo\metre\per\hour} è:

    \makeoption{A} \eqmakebox[first][r]{\SI{36}{km/h}}.     \\
    \makeoption{B} \eqmakebox[first][r]{\SI{360}{km/h}}.    \\
    \makeoption{C} \eqmakebox[first][r]{\SI{27.8}{km/h}}.   \\
    \makeoption{D} \eqmakebox[first][r]{\SI{3.60e8}{km/h}}. \\
    \makeoption{E} \genericoption

  \item In fisica nucleare si usa l'angstrom (simbolo: $\SI{1}{\AA} = \SI{1e-9}{m}$) e il fermi o femtometro ($\SI{1}{fm} = \SI{1e-14}{m}$). 
    Qual è la relazione tra queste due unità di misura?

    \makeoption{A} $\SI{1}{\AA} = \eqmathbox[second][r]{\SI{d5}{fm}}$.   \\
    \makeoption{B} $\SI{1}{\AA} = \eqmathbox[second][r]{\SI{d-5}{fm}}$.  \\
    \makeoption{C} $\SI{1}{\AA} = \eqmathbox[second][r]{\SI{d-15}{fm}}$. \\
    \makeoption{D} $\SI{1}{\AA} = \eqmathbox[second][r]{\SI{d{3}}{fm}}$. \\
    \makeoption{E} \genericoption

\end{enumerate}

\end{document}

I'd suggest using siunitx to set content that has units.

A:

Make the content of all your boxes the same width and don't set your units incorrectly, the spacing and font types are all wrong.
\documentclass{extarticle}

\usepackage{siunitx}

\begin{document}

\fbox{\makebox[0.9em]{A}} $\SI{1}{\angstrom} = \SI{e5}{fm}$.

\fbox{\makebox[0.9em]{B}} $\SI{1}{\angstrom} = \SI{e-5}{fm}$.

\fbox{\makebox[0.9em]{C}} $\SI{1}{\angstrom} = \SI{e-5}{fm}$.

\fbox{\makebox[0.9em]{D}} $\SI{1}{\angstrom} = \SI{e-5}{fm}$.

\fbox{\makebox[0.9em]{E}} \rule{2.2cm}{0.5pt}

\end{document}

