The most stable carbocation for the following is:

Maximum resonating structure are possible in (c) structure. In (a) structure the carbocation is stabilised by lone pair on nitrogen as well as two double bonds in the ring. In (b) and (d) structure lone pair of nitrogen is not involved in conjugation to stabilise positive charge. So, ${\mathrm{NH}}_2$ group will show $-\mathrm{I}$ effect.