What is a base in chemistry?

A base in chemistry is primarily understood as a substance that can neutralize acids, which makes the choice of "a substance that neutralizes an acid" the correct answer. This definition stems from the broader understanding of acid-base reactions where an acid reacts with a base to form water and a salt, effectively reducing the acidity of the solution.

In addition to neutralizing acids, many bases also fit the definition of releasing hydroxide ions when dissolved in water, which aligns with the third choice. However, the key characteristic that distinguishes bases within the context of acid-base chemistry is their ability to neutralize acids.

While the first choice describes the behavior of acids rather than bases, and the last choice pertains specifically to indicators that change color based on pH levels, it does not accurately define a base. Hence, the definition focusing on neutralizing acids captures the essential function of bases within chemical reactions more comprehensively.