Equivalent  weight of an acid: Depends on the reaction involved with a base Depends on the number of oxygen atoms present is always constant none of these

The equivalent weight of an acid is not always constant and it does not depend on the number of oxygen atoms present. Instead, it depends on the reaction involved with a base. This is because the equivalent weight of an acid is defined as the mass of the acid that can supply or react with one mole of protons (H+ ions). In a reaction with a base, the acid donates protons to the base. The number of protons an acid can donate depends on its molecular structure and the specific reaction it is involved in, not on the number of oxygen atoms it contains. Therefore, the correct answer is "Depends on the reaction involved with a base".