A. An element in its pure state is always 0.
B. One of the oxidation rules is that H with a nonmetal (which is Cl) is always +1. Cl would be -2 because -2 + (+1) equals -1.
C. Oxygen is always -2, so Cl- would have to be -1 to equal -3.
It has been awhile since I learnt this in AP Chem so answers could be a little rusty. Hopefully it helps.
