Answer:
A. I was sleeping soundly when the doorbell rang.
Explanation:
Past progressive tense is a form of past tense which indicates the actions that was going on or was happening at a point in the past. The formation of past progressive tense is:
Subject + was/were + past participle of the main verb + -ing + object.
Past progressive action takes place when something was already going in simple past. It indicates a limited duration of the period.