________ is the smallest interval of time after which a periodic motion is repeated.

CONCEPT:

Simple harmonic motion:

• Simple harmonic motion is a special type of periodic motion, in which a particle moves to and fro repeatedly about a mean position.
• In linear S.H.M. a restoring force that is always directed towards the mean position and whose magnitude at any instant is directly proportional to the displacement of the particle from the mean position at that instant i.e. Restoring force ∝  Displacement of the particle from the mean position.

F = -kx

Where k is known as force constant. Its S.I. unit is Newton/meter

EXPLANATION:

1. Frequency: The number of wave cycles happening per second is called frequency.
2. Displacement: It is the change in the position of an object undergoing periodic motion.
3. Amplitude: The maximum amount of displacement of the medium particles from its mean position is called the amplitude of the wave. 
4.  Period: A time period is the smallest interval of time after which a periodic motion repeats itself. Its SI unit is second. Therefore option 4 is correct.