Answer:
2.828 km
Step-by-step explanation:
radius of lake, r = 2 km
Let O be the centre of the circular lake and you walk from A to A to D.
Displacement is the measure of shortest distance between two points.
According to the diagram , the displacement AD is
(By using the Pythagorean theorem)

AD = 2.828 km
thus, the displacement is 2.828 km.