What Is a Child's Language Partner's Purpose in Using Expansion

What is a child's language partner's purpose in using expansion and prompting when interacting with a toddler?

A) To show the child that speaking well does not require the use of sentences that are grammatically correct
B) To increase the child's ability to fastmap words
C) To help the child communicate more effectively by expanding on or asking the child to elaborate on something of interest to him/her
D) To model a problem-solving strategy that the child can use when dealing with complex situations