For what value of $'c'$ will the quadratic equation $ax^2+2bx+c=0$ has equal roots.

Question

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