Complete the dog owner's realization as she looks at her very messy pet.

Look at Buster's muddy paws! He _____ through the neighbor's prize-winning petunia bed again.

The correct answer is must have dug.

We use must have + past participle to show we are almost 100% sure that something happened in the past based on present evidence. The muddy paws are the "clue" that leads to this strong deduction.

Can't have means it's impossible, which contradicts the mud. Should have is for advice or regret, and might dig refers to a present or future possibility.