a b c d

c

$\text{Charges will flow such that final potential of the two spheres become equal} $ $\text{Let q charge flow from smaller sphere to large sphere}$ $V_1 = V_2$  $\Rightarrow \frac{k(Q -q)}{R} = \frac{kq}{2R}$ $\Rightarrow 2Q - 2q = q $ $\Rightarrow q = \frac{2Q}{3}$