Exercise 2.21

The condition (a) holds by the conservation of matter. The condition (b) holds because adding nothing to a reactant leaves you with the reactant as the product. The condition (d) holds because it shouldn’t matter what order reactants are added together in a reaction.

It’s not clear that (c) would hold in all circumstances; adding two reactants may produce a product that no longer reacts with another reactant that the original reactants may have reacted with.