Final Answer: 
The boat has sailed south for 6 km.Thus the correct option is D.
Step-by-step explanation:
To determine the distance the boat has sailed south, we can employ basic trigonometry using the concept of vectors. If the boat has sailed directly south, the distance traveled can be found by considering the displacement in the south direction. If the boat has moved both east and south, forming a right-angled triangle, the distance sailed south can be found using the Pythagorean theorem (option D).
Assuming the boat sailed both south and east, forming a right-angled triangle, and considering the Pythagorean theorem 
 is the hypotenuse and
 is the hypotenuse and 
 are the other two sides of the right-angled triangle, the distance south
 are the other two sides of the right-angled triangle, the distance south 
 can be calculated using the given distances. If the boat traveled 3 km east and 4 km south, the distance traveled south can be calculated as
 can be calculated using the given distances. If the boat traveled 3 km east and 4 km south, the distance traveled south can be calculated as 
 . However, if the boat traveled an additional 1 km east and 2 km south, the total distance traveled south becomes
. However, if the boat traveled an additional 1 km east and 2 km south, the total distance traveled south becomes
 which is closer to 6 km than to 5 km, suggesting the boat has sailed a total distance of 6 km south.(option D).
 which is closer to 6 km than to 5 km, suggesting the boat has sailed a total distance of 6 km south.(option D).