John Gill Commentary Mark 9:6

"For he knew not what to answer; for they became sore afraid." — Mark 9:6 (ASV)

For he wist not what to say
He did not know what he should say, or what was proper to be said by him, at such a time, in such circumstances, and before such persons;

for they were sore afraid .
The Persic version reads, "he was": and so the Latin translation of the Syriac, though that itself is, "they were"; for all three were filled with consternation at what they saw and heard; so that they were scarcely themselves, and did not know well what they said or did.