The old man was sitting . . . . . . . . the corner of the room.

A: at - This is the most suitable preposition to use when describing someone sitting in a specific point within a larger area like a room. Think of "at" as indicating a precise location.

B: in - We generally use "in" when something is enclosed or surrounded by something else. For example, "in the box" or "in the garden." While technically possible if the corner was a very defined, enclosed space, "at" is more common and natural.

C: on - "On" typically means on top of a surface. It wouldn't make sense to sit "on" the corner of a room unless the corner was a physical object you could sit on, like a piece of furniture.

D: across - "Across" indicates movement or a position on the opposite side of something. It doesn't describe sitting in a corner.

Therefore, "The old man was sitting at the corner of the room" is the most grammatically correct and natural-sounding option.