ANSWER
D. 4km [E]
Step-by-step explanation
The displacement is the distance between the start and ending points of the object's path. If we draw the boat's path,
Since the boat moved in opposite directions, the total displacement is the difference,
![15\operatorname{km}-11\operatorname{km}=4\operatorname{km}]()
Because the result is positive, the displacement is in the direction of the first part of the trip: 4km [E]