You can't imagine_____ great fun chatting online is A what B how 选哪个?为什么?

You can't imagine_____ great fun chatting online is A what B how 选哪个?为什么?
You can't imagine_____ great fun chatting online is A what B how 选哪个?为什么?

You can't imagine_____ great fun chatting online is A what B how 选哪个?为什么?
You can't imagine_____ great fun chatting online is.
选A what
great fun是名词短语,所以用what

what
对名词fun感叹

A,因为fun是不可数名词，应与what相搭配