You Are Working on a System Issue at a Client's

You are working on a system issue at a client's site and notice that the resolution is taking you longer than you anticipated. As a result, you know you will be late to your next appointment. What is the best way to deal with this situation?

A) Continue working on this issue. You'll get to the next client when you get there.
B) Tell the client at your current site that the issue is more in depth than you anticipated and offer to return after hours.
C) Call your next appointment and tell them you are running behind while fixing another issue and offering to reschedule if they wish.
D) Tell the client at your current site that the issue is more in depth than you anticipated and they will need to schedule another day to have it resolved.