Hello. Although you have shown that this question is about the text "Earth's Core" you have not shown the passage to which the question refers, this makes it impossible for this question to be answered. However, I will try to help you in the best possible way.
A dialogue is a conversation presented by two or more characters, what propels the dialogue is the subject that is being discussed during the conversation and the way the characters are talking. Thus, we can consider that what propels the conversation, that is, what motivates the conversation to happen, may be, a disagreement between the characters on the same subject, a fear presented by the characters, a concern or hope that the characters have, or even a description of something they’re seeing.
Thus, to answer this question you will have to read the dialogue and identify the theme and the way the characters are talking.