ANS:A - number of electrons.

An oxidation process is accompanied by a decrease in the number of electrons and an increase in the oxidation number. When a substance undergoes oxidation, it loses electrons, which results in an increase in its oxidation number. For example, in the reaction: Mg→Mg2++2e− Magnesium (Mg) loses two electrons to form magnesium ions (Mg2+Mg2+). In this process, the oxidation number of magnesium increases from 0 to +2, and the number of electrons decreases by 2. So, the correct option is (a) number of electrons and (b) oxidation number.