The length of the lake is the side labeled L. The corresponding side in the other triangle is 45 m. The side labeled 90 m corresponds to the side labeled 20 m.
We have two corresponding side we know, and one of the other two corresponding sides, so we can setup a proportion and solve fro L.
90/20 = L/45
4.5 = L/45
202.5 = L
L = 202.5
Answer: C. 202.5 m