A.T. Robertson Commentary Matthew 21:24

"And Jesus answered and said unto them, I also will ask you one question, which if ye tell me, I likewise will tell you by what authority I do these things." — Matthew 21:24 (ASV)

One question (λογον ενα). Literally "one word" or "a word." The answer to Christ's word will give the answer to their query. The only human ecclesiastical authority that Jesus had came from John.