A.T. Robertson Commentary Mark 11:28

"and they said unto him, By what authority doest thou these things? or who gave thee this authority to do these things?" — Mark 11:28 (ASV)

By what authority (εν ποια εξουσια). This question in all three Gospels was a perfectly legitimate one. See on Mt 21:23-27 for discussion. Note present subjunctive here (ινα ταυτα ποιηις), that you keep on doing these things.