The distribution function of the random variable X is F(t) = 1 − e−5t3

, t ≥ 0.

The density function of X (for x ≥ 0) is

(a) f (x) = e−5x2

(b) f (x) = 5e−5x2

(c) f (x) = 15e−5x3

(d) f (x) = 5x2e−5x3

(e) f (x) = 15x2e−5x3