One of the diagonals of a rhombus is double the other diagonal. Its area is 25 sq. cm. The sum of the diagonal is ?

Answer: Option C

$$\begin{array}{rl}& \iff \frac{1}{2}{d}_1\times 2d_1=25\\ & \iff d_1^2=25\\ & \iff d_1=5\end{array}$$
∴ Sum of lengths of diagonals = (5 + 10) cm = 15 cm