challenge (one) on (something)
To question one, perhaps aggressively, on a particular issue, statement, or viewpoint. I had to challenge him on that remark about sales because the budget report did not support it at all. That is a positively absurd statement, and I'm glad someone challenged her on it. Challenging someone on their political beliefs the first time you meet them is usually not the best idea.
challenge someone on something
to dispute someone's statement, remarks, or position. I think Fred is wrong, but I won't challenge him on his estimate.