The line is 彼はある観客からの予想外の質問にたじろいだ。but since たじろぐ is a verb (and since it’s clearly translated as past tense), I think this was meant to be a た not a だ.
たじろいだ is correct since the verb ends in ぐ. See Japanese Verb Plain Past た Form
Ah, now I feel silly. I forgot how that works or I read it as く or something.