The mean of an f(x) function is equal to the integral of that function's product to it's respective x on the given range. For the function f(x)= x for 0 ? x ? 3 the integral of x*f(x) on the range 0 to 4 is taken. This would yield to the integral of x² which is equal to x³/3. For x=0 this would equal to 0, for x=4 this would equal to 9. The mean is equal to 9 - 0 = 9. The answer is B.