If $ax^2+bx+c=0$ has equal roots ,then $c=$_______

Question

$\begin{array}{1 1}(A) \; \large\frac{-b}{2a}\\(B)\;\large\frac{b}{2a} \\(C)\;\large\frac{-b^2}{4a} \\(D)\;\large\frac{b^2}{4a} \end{array}$