What does "forerunner" refer to?

The term "forerunner" specifically refers to something that precedes or indicates the approach of another person, object, or event. In many contexts, it can denote a predecessor that serves as a sign or signal of what is to come. For example, in a scientific context, a forerunner could be an earlier species that hints at the development of a new one, or a trend that presages future developments in a particular field. This understanding aligns well with the essential meaning of the term and captures its usage in various contexts, including literature, science, and everyday language.

The other options do not fit the definition of "forerunner." A successor is someone or something that comes after another, and a component of a larger assembly refers to parts that make up a whole, which doesn’t align with the concept of something that precedes or indicates. Meanwhile, a type of measurement does not relate at all to the idea of anticipation or indication of something on the way. Hence, the identification of "forerunner" as one that indicates the approach of another is substantiated by common usage and contextual understanding.