Find the value of p for which $e=\large\frac{1}{2}$ where ellipse is $\large\frac{x^2}{4}+\frac{y^2}{p^2}$$=1$?

Question

$\begin{array}{1 1}(a)\;p=\sqrt 3,\large\frac{4}{\sqrt 3}\\(b)\;p=\sqrt 5,\large\frac{4}{\sqrt 5}\\(c)\;p=\sqrt 7,\large\frac{4}{\sqrt 7}\\(d)\;\text{None of these}\end{array}$