This is a check of your integrity. If you are attending an interview during working hours, you have to take off from your current company. So tell the simple answer.
You are not expected to tell whether you have told your current manager about taking off for an interview. So do not get confused here in telling extra details.
Sample answer is:
I have taken off from work for a few hours today to attend this interview. I have already finished my deliverables yesterday, so that I can be free to attend the interview today.