Mark 11:29
And Jesus answered and said unto them, I will also ask of you one question, and answer me, and I will tell you by what authority I do these things. And Jesus answered and said unto them, I will also ask of you one question, and answer me, and I will tell you by what authority I do these things.
When challenged by the rulers over His authority to cleanse the temple, Jesus answered with a question of His own and made His telling them by what authority He did these things conditional on their answering Him. He asked whether John's baptism was from heaven or from men, that is, from human authority. Their answer would reveal whether they accepted John as a prophet and, in turn, whether they would accept John's testimony identifying Jesus as the Messiah.
In this way the question was part of the evidence given to an unbelieving people of His divine mission. They professed ignorance and refused to answer, because either response would expose their rejection of John's message or risk the people's anger and threaten their standing with the people. Jesus then declined to tell them His authority, so their evasiveness left them unable to demand an answer from Him and exposed their cowardice and indecision.