(a) In group 12 of periodic table B is metal and In group 17 A and C is nonmetal and halogens.

(b) A is more reactive than C because, We take A as Cl and C as Iodine.Now,

• Both have 7 electrons in its outer shell and require 1 electron to obtain a full shell. Reactivity of these halogens correspond to its ability to gain an electron. 
• Cl can gain an electron more easily than iodine - Cl more reactive. This is because Cl is higher up Group 7 than iodine (Cl: 2,8,7 electron configuration).

(c) B is larger than A in periodic table because,

• In general, the size of an atom will decrease as you move from left to the right of a certain period.

(d) A and C ( A and C belongs to halide family ) mainly more occurs as anion  and B (belongs to Zn family) occurs as both in given condition because,

• Atoms of group 17 gain one electron and form anions with a 1− charge; atoms of group 16 gain two electrons and form ions with a 2− charge, and so on.
•  For example, the neutral bromine atom, with 35 protons and 35 electrons, can gain one electron to provide it with 36 electrons