PhD: Should I choose a topic out of a comfort zone? I am accepted to a math PhD program and have a visit in a few weeks. I only have a few days to tell the university which professors I would like to meet.

Problem: I also still don't know what I would like to work on (which I stated explicitly in my SOP) and the departments has ~100 faculty members. I started to go one by one and see if I like what they do but what would make a good topic?

Reading the papers from professor A things look "easy", I don't know the solution to the problems but I'm in familiar territory: it is number theory, a junior high school could understand the problem, and maybe high school could understand some solutions.

The vast majority of the time, for other professors, I don't understand anything at all. Sometimes I have a clear disgust (seeing lots of integrals/computations), sometimes I feel it is too hard (Langland related algebra), ...

I wonder how discomfortable I should be with a subject? I would be sad to end a PhD with seemingly "no additional knowledge" (if I choose prof. A) but conversely I would also be sad if I struggle on something too hard for me or for which I have no appetite even after spending time understanding it. Finally I don't have enough time to spend hours on each professor's topic to see if I would like it.

What is a good way to approach this?
Pick the ones who interest you to meet with.  Probably some function (topics I like, big names).  I.e. y= f(x,t).

Who you meet with is not going to make your final decision on what school to go to.  But for sure you need to meet with those who you think are seriously "in zone" for being your advisor.  You need that scoop.  I would even make a stink if you don't meet with a key guy (their time is limited but so is yours...you are only making on visit...have seen candidates get a breakfast or lunch meeting or at least meet with grad student representative).

In terms of what work you should do, I strongly recommend to do something you are confident in.  This stuff is hard enough so you need an edge.  Also make it something you are interested in.  This stuff is hard enough so you need some enjoyment.  See.  Multifactorial.  Not f(x) versus f(t) but f (both).  maximize, maximize.  but each variable needs to be above average.  Say above 7 on a 0 to 10 subjective.  Don't let a 10  on one allow a 5 on the other.