What does the man say when he is asked to borrow a pen?

The statement "He hasn't got one" is correct because it indicates that the man does not have a pen available to lend. This response directly addresses the request to borrow a pen, showing that he is unable to assist the person asking. It suggests a clear understanding of the question and conveys a straightforward answer regarding his possession.

The other options do not appropriately respond to the request in the same way. The choice indicating that he has one would imply he can lend it, while saying he will find one suggests he might be willing to help but doesn't currently have a pen on hand. Claiming he is using it means he has a pen, but it is not available for borrowing. Thus, the correct answer effectively communicates that he does not possess a pen to lend at the moment.