You should ask questions in a group discussion to

illustrate the points you want to make with examples

contribute your own information and ideas

relate discussion points to your own experience

enhance your understanding of another speaker’s ideas

You should ask questions in a group discussion to enhance your understanding of another speaker's idea.

A group discussion is basically aimed at sharing views and opinions among a group of individuals. If the purpose is to exchange information and opinions about a topic, then asking questions to understand the other's point of view is essential.

The rest of the answers (A,B and C) limit the possibility of exchanging ideas, rather than fostering it.

modifying the base for word