C

Step 1: When a body of mass m is in a lift accelerating upwards, the forces acting on it are its weight (mg downwards) and the normal reaction force (R upwards) from the floor of the lift.

Step 2: According to Newton’s second law, the net force acting on the body is equal to the mass times its acceleration (F = ma).

Step 3: The net force on the body can also be written as the difference between the normal force R and the weight mg:

\[ R - mg = ma \]

Step 4: Rearranging this equation gives:

\[ R = mg + ma \]

Hence, the correct equation of motion is represented by Option C: \( R = mg + ma \).
Therefore, C.