The variance is equal to V (X) = E (X²) - [E (X)]² for f(x) = x for 0 ? x ? 4 the f(x²)=x² which would indicate that x*x² is the function to take the integral in order to find the variance. The integral of x³ is x⁴/4 on 0 ? x ? 4 would yield to the difference of the value found from x = 4 and x = 0. For x=4 the value is 64 and for x=0 the value is 0. The difference between these values is 64. The answer is E.