The correct answer is "A is true, but R is false."

Assertion (A) states that in a motion picture, usually 24 frames are projected every second over the whole length of the film. This is true and refers to the standard frame rate used in motion pictures. The frames are projected rapidly one after another to create the illusion of continuous motion.

Reason (R) states that an image formed on the retina of the eye persists for about 0.1 seconds after the removal of the stimulus. This is false. The persistence of vision refers to the phenomenon where the human eye retains an image for a brief moment after the stimulus is removed. However, the persistence of vision is typically around 0.04 to 0.1 seconds, depending on various factors. It is not directly related to the frame rate of a motion picture.

Therefore, while Assertion (A) is true, Reason (R) is false. The correct answer is "A is true, but R is false."