Please answer fast...The answer is (1).
Q. An electron is projected normally from the surface of a sphere with speed $v_0$ in a uniform magnetic field perpendicular to the plane of the paper such that its strikes symmetrically opposite on the sphere with respect to the x-axis. Radius of the sphere is 'a' and the distance of its centre from the wall is 'b'. What should be magnetic field such that the charge particle just escapes the wall :-

(1) $B=\frac{2bm{v}_0}{\left(b^2-a^2\right)e}$
(2) $B=\frac{2bm{v}_0}{\left(a^2+b^2\right)e}$
(3) $B=\frac{m{v}_0}{\left(\sqrt{b^2-a^2}\right)e}$
(4) $B=\frac{2m{v}_0}{\left(\sqrt{b^2-a^2}\right)e}$