Verse 29. - I will ask of you one question (ἐπερωτήσω ὑμᾶς ἕνα λόγον). The verb justifies the translation, one question, for "one word." The question which our Lord put to them was one on which hung the solution of that proposed by the scribes. It is as though he said, "You do not believe me when I say that I have received power from God. Believe then John the Baptist, who bare witness of me that I was sent from God to do these things."
11:27-33 Our Saviour shows how near akin his doctrine and baptism were to those of John; they had the same design and tendency, to bring in the gospel kingdom. These elders did not deserve to be taught; for it was plain that they contended not for truth, but victory: nor did he need to tell them; for the works he did, told them plainly he had authority from God; since no man could do the miracles which he did, unless God were with him.
I will also ask of you one question; or "one word", or "one thing"; here answers to the Hebrew word which signifies both "word" and "thing":
and answer me, and I will tell you by what authority I do these things; See Gill on Matthew 21:24.