The correct option is
A That's OK. I'm not in a hurry.
Here, we have to find the best response for the given sentence. If someone asks us to wait, who do we say? We generally respond politely. Let's have a look at the options.
Option B - 'Oh! I would love to.' is a polite response, but we never love to wait. This is an inappropriate response. Hence, option B is incorrect.
Option C - As explained earlier, we usually respond politely to our superiors or our colleagues. 'That isn't very nice' is a bit rude. Hence, option C is incorrect.
Option D - 'That's a good thing' is logically incorrect, as 'waiting' is never a good thing. Also, this response seems to be a little sarcastic. Hence, option D is incorrect.
Option A - Yes, 'That's OK. I'm not in a hurry' is a comforting and a polite response. Hence, option A is correct.