All we need to find is the angle of elevation of the sun. In the figure, AB is 5m long stick and BC is 2m long shadow of the stick. We join A and C to get a right-angled triangle ∆ABC. Angle of elevation of the sun is ∠ACB.

In ∆ABC,

or,

In this figure, PQ is the tree and QS is its shadow. ∠PSQ is the angle of elevation of the sun. So, ∠PSQ = ∠ACB. Join P and S. We get a right-angled triangle ∆PQS. Given, PQ = 12.5m. We use trigonometric ratio tan taking PQ as the height and QS as the base.

In ∆PQS,

or,

or,

So, the correct option is (D).