The teacher often tried to get his quieter students to elaborate on their ideas by asking them concrete questions which served to flush out minutiae and descriptions.
Which of the following substitutions best captures the meaning of the underlined words?

To elaborate means to develop an idea more clearly. The addition of the preposition "on," used to make the verb transitive, gives a nuance to the phrase that begs the question of how one elaborates. In this case, adding detail to what is said allows the ideas to become more fully developed.