(a)

we can see that
curve is moving upward from t=0 to t=4
so, this is increasing interval.........Answer
(b)

we can see that
curve is moving upward from t=15 to t=18
so, this is increasing interval.........Answer
(c)

we can see that
curve is moving downward from t=4 to t=15
so, this is decreasing interval.........Answer