Select the grammatically correct phrase to complete the mad scientist's diary entry.

Looking at the giant, radioactive hamsters currently destroying the city, I realize that I _____ the growth serum with my morning espresso.

The correct answer is ought not to have mixed.

"Ought to" is a marginal modal used to express advice or regret. To express regret about a past action, we use ought to have + past participle.

In the negative, "not" is placed between "ought" and "to" (ought not to have done).

"Didn't ought" and "hadn't ought" are non-standard grammatical errors because "ought" does not take auxiliary verbs like "do" or "have" for its negation. "Ought not to mix" is incorrect because it refers to the present or future, not a completed past action.