The condition $f(x)=x^3+px^2+qx+r(x \in R)$ to have no extreme value, is

Question

$$\begin {array} {1 1} (a)\;p^2 < 3q & \quad (b)\;2p^2 < q \\ (c)\;p^2 < \frac{1}{4} q & \quad (d)\;p^2 > 3q \end {array}$$