Corresponding in size or amount to something else.
D:
When electrons are gained, the charge of the atom decreases.
When you are given an atom with a charge, the oxidation of that atom is the charge. So by going from a Cr^3+ (Oxidation Number = 3) to a Cr^2+ (Oxidation Number = 2), the Oxidation Number thus decreases.
I'm pretty sure your answer should be CaCl2. Hope this helps! =^-^= (if I need to explain more just let me know.)