What important question does Ralph ask Jack? Explain significance of Jack's silence as his response.In Chapter 7.

I think the question you are referring to is asked as they are heading across the island to find Piggy.  At one point, Ralph asks Jack why Jack hates him.  As you say, Jack remains silent.

This is an important question and answer because it brings up a couple major points in the book.  First, it reminds us that Jack hates Ralph, though that was clear already.  Second, Jack's silence implies that he does not have a reason.  It implies that some part of human nature just naturally hates order and rules.  That part of human nature (which Golding says is dominant) wants to be vicious and brutal like Jack.