Which sentence indicates that a past event was in progress when it was interrupted?
rockwell had painted in his studio when a fire broke out and destroyed everything.
rockwell was painting in his studio when a fire broke out and destroyed everything.
rockwell painted in his studio when a fire broke out and destroyed everything.
rockwell had been painting in his studio when a fire broke out and destroyed everything.

The correct answer is the sencond option. In order to convey the meaning that an action was taking place but it was interrupted by another action, you need to use the past continoues tense. This past tense in combination with the past simple has exactly that meaning of action being interrupted, by another past action. In this case the action of "painting" stopped because of the fire "broke out and destroyed everything". The fourth option has a similar meaning, but it is not exactly that of interruption