Select the correct verb form to complete the pet owner's funny observation.

When I finally found Buster in the garden, he was covered in mud and looking very guilty. He ___ a massive hole under the rosebushes all morning!

The correct answer is had been digging.

The past perfect continuous (had been digging) is perfect here! It describes a continuous, ongoing action that happened before another past action (finding Buster). The time phrase "all morning" tells us how long the muddy mischief lasted before he was caught.