The correct answer is: B. was sitting
The simple past tense is used to talk about actions that happened at a specific time in the past. The past continuous tense is used to talk about actions that were happening at a specific time in the past.
In this sentence, the time is not specified, so the simple past tense is not appropriate. The past continuous tense is more appropriate because it describes an action that was happening at a specific time in the past.
The other options are incorrect because they are not in the past continuous tense.