Sadly, none of these are correct.
A. All of us seated, the meeting began.
B. All of us had taken the seats, the meeting began.
C. All of us have been seated, the meeting began.
D. All of us having seated, the meeting began.
They all would need to be modified to be correct. For example:
A. With all of us seated, the meeting began.
B. After all of us had taken the seats, the meeting began.
C. When all of us have been seated, then the meeting can begin.
D. With all of us having seated, the meeting began. (This one is a bit of a stretch - better as Having seated ourselves, ...).
I suspect what they might have been looking for is agreement in the tense. As began is the simple past / imperfect tense, the phrase that also has simple past is A. All of us seated, as the verb to seat conjugates as seat / seated / seated.
A. seated is simple past
B. had taken is past perfect
C. have been seated is passive / indicative present perfect
D. having seated is the past participle.
So, the answer is most likely given as A because of the tense agreement. But it is not good grammar.