A) The set Σ* is countable because each element of this set can be generated in the following order (called proper order):
Let Σ={a,b}. So, proper order = a,b,aa,ab,ba,bb,aaa,aab....
D) The set of all languages accepted by TMs is the set of all TMs basically, which is countable because each TM can be represented by a binary string and each binary string can be obtained in a proper order (as stated above) and checked whether it's a TM or not.
C) The set of all regular languages is a subset of the set of all recursively enumerable languages. And a subset of a countable set is always countable. This is because all the elements of the countable set can be written in a specific order and each of that element can be checked for its membership in the other set.
B) The set of all languages is uncountable, according to Cantor's Diagonalisation Proof.