Answer:
The correct answer to your question is letter A
Step-by-step explanation:
a. [ Ca: ] This structure is correct because Calcium has 2 valence electrons and in this structure we can see them as points to the right side of the symbol.
b. [ Ca ]2+ This option is not correct, in Lewis dot representation, electrons are points and this structure do not show them.
c. No right choice. This option is wrong because letter A is correct.