Choose the correct phrase to complete the detective's dramatic memoir.

Only when Inspector Clouseau tasted the crumbs _____ that the deadly weapon was, in fact, a slightly stale baguette.

The correct answer is did he realize.

When a sentence begins with "Only when" followed by a subordinate time clause ("Inspector Clouseau tasted the crumbs"), the subject-auxiliary inversion is delayed. It must occur in the main clause ("did he realize"), not the subordinate clause. "Did he realized" is incorrect because the auxiliary "did" already carries the past tense, so the main verb must be in its base form.