The ancient Roman forum, a bustling center of political and social life was home to many important public buildings.
Correct: B
The phrase 'a bustling center of political and social life' is an appositive that provides additional, non-essential information about 'the ancient Roman forum.' Appositives should be set off by commas if they are non-essential. Since the first comma is already present, a second comma is needed after 'life' to complete the parenthetical phrase. Option B correctly places this comma.