“Sure. What's up?”

One of the people who works under you came to your office to ask a question. He asked you "Can I ask you something?" You were typing an email, but it wasn't that important, so you answer:

Sure. What's up?

Sure.

As I've explained before, "Sure." is a casual way of saying "Yes, of course." It sounds friendlier than just answering "yes" to a question.

What's up?

The phrase "What's up?" has many different uses, but one of them is to respond when someone asks to talk with you:

A: Hey, can I talk with you for a minute?

B: Yeah, what's up?

"What's up?" sounds light and casual. You can use this with people who you work closely with, who you're friends with, or who work for you.

A somewhat more formal answer to this question is:

Sure, what can I do for you?

Or, if the person you're talking to seems upset, you can ask:

What's going on?