The mas is sitting -- present tense, progressive/continuous aspect, so:
The man was sitting...past tense, progressive/continuous aspect. - this your answer.
other past tenses:
The man sat. simple past
The man has sat....present (tense) perfect (aspect) -- but the present perfect is treated as a past tense in English.
The man had sat....past tense, perfect aspect.
The man did sit. past tense, emphatic aspect.
The man has been sitting -- present perfect progressive/continuous (present tense, perfect and progressive/continuous aspects).
The man had been sitting -- past perfect progressive...
If the only thing you are doing is changing to the past tense, then only "was sitting" is correct, from "is sitting". Any other form changes the tense, aspect, or both.