Answer
$x-\dfrac{x^3}{3}-\dfrac{x^6}{2}+C$
Work Step by Step
Since, we have $\int (1-x^2-3x^5) dx=x-\dfrac{x^3}{3}-\dfrac{x^6}{2}+C$
Also, $\dfrac{d}{dx}(x-\dfrac{x^3}{3}-\dfrac{x^6}{2}+C)=1-x^2-3x^5$
Thus, the derivative of the integral is integrand. So, the answer is correct.