Choose the correct phrase to finish the time traveler's diary entry.

When I found my smartphone buried in the Jurassic soil, I realized it ______________ by a dinosaur, which explains the massive, three-toed scratches on the screen.

The correct answer is must have been stepped on.

This sentence requires the past modal passive form (must have been + past participle) to express a strong logical deduction about an action performed on the subject in the past.

"Ought to have stepped on" is in the active voice and implies an unfulfilled obligation. "Should have been stepped on" implies it was a good idea for the dinosaur to crush the phone. "Can have been" is grammatically incorrect here; we use could have or must have for positive past deductions, never can have.