People at a lecture or concert

Answer: Option B

One word substitute is Audience.

Congregation: a group of people assembled for religious worship.
Audience: the assembled spectators or listeners at a public event such as a play, film, concert, or meeting.
Mob: a large crowd of people, especially one that is disorderly and intent on causing trouble or violence.
Spectators: a person who watches at a show, game, or other event.