A point charge Q is placed at the centre of an imaginary Gaussian surface. Find flux through curved surface.

Question

 

$(A)\;\frac{al}{2 \in _0 \sqrt {\frac{l^2}{4} +R^2}} \\ (B)\;zero \\ (C)\; \frac{Q}{2 \in_0 }\bigg[ 1- \frac{l}{2 \sqrt {R^2+\frac{l^2}{4} }}\bigg] \\ (D)\;\frac{Q}{\in_0 }\bigg[ 1- \frac{l}{2 \sqrt {R^2+\frac{l^2}{4} }}\bigg]$