Answer:
The answer would be A. which means Sorry, but I am working.
Step-by-step explanation:
If you were to pick B. which means "of course" you would be saying you want to come which would not make sense.
If you were to pick C. which means "that's nice of you" which would still work but if you wanted to say you didn't want to come not the right choice.
If you were to pick D. which means " I regret" which would not make sense at all sense that is more past tense.
So A. would be the only one that makes sense.