The angle of elevation of the sun, when the length of the shadow of a tree is equal to the height of the tree, is:

Answer: Option C

Consider the diagram shown above where QR represents the tree and PQ represents its shadow
$$\begin{array}{rl}& \text{We}\text{have},QR=PQ\\ & \text{Let}\angle QPR=\theta \end{array}$$
$$\tan \theta =\frac{QR}{PQ}=1$$     (since QR = PQ)
$$\Rightarrow \theta =45^{\circ }$$
i.e., required angle of elevation = 45°