Work done by gravity is

A. Positive when the body is moving down

B. Negative when the body is moving up

C. Zero when the body is moving horizontally

While moving down, the gravitational force and displacement both are in same direction. Thus, work is positive.

While moving up, the gravitational force and displacement are in opposite direction. Thus, work is negative.

While moving horizontally, the gravitational force and displacement are perpendicular to each other. Thus, work is zero.