Hi!
That's a funny one xD
The correct answer is A
Tell them you are busy now and suggest an alternative time to talk.
The other options are not good because if you just tell them you are bus now, you will lost the client. They will think you are not care for them. You can't tell them to make an appointment with your assistant because they want to talk with you,not the assistance. OMG always keeping the door closed is the worst xD. It shows that you don't welcomed people so you wouldn't have enough client.
I hope this helps!