Final answer:
A base accepts a hydrogen ion (proton) when it reacts with another substance, according to the Brønsted-Lowry theory of acids and bases.
Step-by-step explanation:
The statement that is true about a base when it reacts with another substance is that it receives a hydrogen ion. According to the Brønsted-Lowry theory, a base is defined as any substance that can accept a proton (hydrogen ion, H+). The most common definition of a base includes substances such as hydroxide ions (OH-) that react with protons to form water, but the Brønsted-Lowry definition is more general and includes any proton acceptor. Therefore, when a base reacts, it is involved in a transfer of protons where it acts as the acceptor, leading to the formation of its conjugate acid.