"Are you able to make frequent business trips?"
"Are you willing to travel?"

If they are asking this, then this position probably requires some traveling. If you say no, then you probably won't get hired because they want someone who is able to travel. But depending on your job, you will already know if it requires traveling or not.

"I know as an auditor that I'll have to make business trips. So I'm definitely willing to travel."

"I don't have any problems with traveling."