Explanation

(a) An aeroplane files $400\ m$ north and $300\ m$ south so the net displacement is $100\ m$ towards north
Then it files $1200\ m$ upwards so $r = \sqrt{(100)^{2} + (1200)^{2}}$
$= 1204\ m \simeq 1200\ m$
The option should be $1204\ m$, because this value mislead one into thinking that net displacement is in upward direction only