Fig. 14.23 depict

A periodic motion is the one performed by particles oscillating back and forth at regular intervals.

Here, in x-t plots it is seen,

(a) As the time proceeds, the particle continues to proceed with increasing distance without oscillating. Hence, the motion is non-periodic. (Refer Fig 14.23-(a))

(b) The particle vibrates back and forth continuously, producing a periodic motion with a time period of 2 seconds. (Refer Fig 14.23-(b))

(c) The motion is non-periodic as seen in the graph. (Refer Fig 14.23-(c))

(d) The motion is periodic with a period of 2 seconds. (Refer Fig 14.23-(d))