A swimmer is determined to cross a river that flows due south with a strong current. Initially, the swimmer is on the west bank desiring to reach a camp directly across the river on the opposite bank. In which direction should the swimmer head?

(A) The swimmer should swim southeast.
(B) The swimmer should swim south.
(C) The swimmer shuold swim northeast.
(D) The swimmer should swim due north.
(E) The swimmer should swim due east.

C)

Explanation:

As velocity is a vector, if he tried to swim directly due east, his total velocity, would be the vector sum of his own velocity (towards due east) and the one due to the  current (due south), so he will be aiming to the south-east direction.

In order to counteract this, he needs to aim in such a way, that when adding (vectorially) his own velocity to the one due to the current, gives a resultant vector aiming due east, so, he needs to swim northeast.