Answer: It's A
Reason:
Calcium has an oxidation number of +2 that makes (Ca)
Cyanide has an oxidation number of -1 so it needs a base of 2 in parentheses
The 2 is to multiply (-1 x 2 = -2) while Ca is already +2 so it needs no base
-2+2=0
FINAL ANSWER IS Ca(CN)2