What's the answer of (10+13) ≡? As to Modulus operation I only have seen this form:
(x + y) mod z  ≡ K

So I can't understand the question, by the way the answers are :
a) 8 (mod 12)
b) 9 (mod 12)
c) 10 (mod 12)
d) 11 (mod 12)
e) None of the above


The OP may be taking a Computer Science course, in which if $b$ is a positive integer, then $a\bmod{b}$ is the remainder when $a$ is divided by $b$. In that case  $\bmod$ is a binary operator. That is different from the  $x\equiv y\pmod{m}$ of number theory, which is a ternary relation (or, for fixed $m$, a binary relation).  
Calculating $(10+13)\bmod{12}$ is straightforward. Find $10+13$, and calculate the remainder on division by $12$. We get $(10+13)\bmod{12}=11$.  So $(10+13)\bmod{12} \equiv 11\pmod{12}$. 
Remark: It seems unusual to use the binary operator and the ternary relation in a single short expression.
