Complete the polar explorer's dramatic memoir.

_____ the sudden blizzard, we would have safely reached the summit by noon.

The correct answer is Had it not been for.

This is a third conditional sentence describing an unreal past situation ("would have reached"). To invert a negative third conditional phrase like "If it had not been for," we drop if and move the auxiliary verb had before the subject it.

The negative word not remains after the subject. Therefore, "Had it not been for" is the only grammatically correct choice. "Were it not for" and "If it were not for" refer to the present or future (second conditional).