Final answer:
OPTION A.The swimmer should swim upstream at an angle against the flow to cross the river straight, taking into account the current of the river and their own swimming speed.
Step-by-step explanation:
The direction the swimmer should take to cross the river straight is against the flow of the river, which is upstream. When a swimmer wants to cross a river and not drift downstream, they must swim at an angle to the current. This angle should be calculated such that the swimmer's effective velocity across the river is directly perpendicular to the riverbanks. In this scenario, since the swimmer's speed is greater than the speed of the river flow (4 km/h swimmer speed vs.
2 km/h river flow), the swimmer can indeed cross the river straight by swimming upstream, but it requires precise angling to ensure that the current's effect does not push them downstream. Swimming straight upstream or straight downstream would not accomplish crossing in a straight line, and the correct direction must account for the current of the river and the swimmer's own speed. Swimming at an angle against the flow allows the swimmer to compensate for the current and move towards the direct path across the river.