Complete the detective's dramatic report about the great museum heist by selecting the grammatically correct phrase.

When the security guard finally woke up from his nap, the priceless diamond ___ into the back of a getaway van!

The correct answer is was being loaded.

We use the past continuous passive (was being loaded) to describe an ongoing action in the past that was happening at the exact moment another event occurred (the guard waking up). The diamond wasn't loading itself, so the active forms ("was loading" or "loaded") are incorrect!