What is the formula for arsenite?

The formula for arsenite is AsO3^3-. Arsenite refers to the arsenic species with a +3 oxidation state. In this case, arsenite ions consist of one arsenic atom bonded to three oxygen atoms, and the overall charge on the ion is -3.

This is in contrast to other choices listed. For example, the formula AsO4^3- would represent arsenate, which is a different species with arsenic in the +5 oxidation state. The other formulas, BO3^3- (borate) and PO4^3- (phosphate), do not have arsenic in them at all, so they are unrelated to arsenite. Thus, the correct identification of AsO3^3- as the formula for arsenite is based on its composition and the oxidation state of arsenic within that ion.