The correct answer is D) KLIS, DE, his own call letters. When you have called another vessel by flashing light and they have answered properly, the appropriate response for them is to repeat your call sign (KLIS, DE) followed by their own call letters. This is the standard procedure for responding to a flashing light call under the International Code of Signals and Coast Guard regulations. The other options are incorrect because: A) TTTT is not a proper response, B) just responding with 'R' is too brief, and C) the other vessel should not just repeat your call sign without including their own.