Help the clumsy time traveler write his apology log. Choose the correct phrase to complete his sentence.

If only I _______ that giant red button during the dinosaur era. Now we have T-Rexes roaming the streets!

The correct answer is hadn't pressed.

To express regret about something that happened (or didn't happen) in the past, we use if only or wish followed by the past perfect tense (had + past participle). Since he actually pressed the button in the past, he uses the unreal past perfect "hadn't pressed" to wish the opposite.