The equivalent single discount for two successive discounts of 15% and 10% is = ?

Answer: Option C

Equivalent to a single discount
$$\begin{array}{rl}& =15+10-\frac{15\times 10}{100}\\ & =25-1.5\\ & =23.5\%\end{array}$$

Alternate :
$$\begin{array}{rl}& \text{Market price = Rs}\text{.100}\\ & \text{Cost price}\\ & \text{ = 100}\times \frac{85}{100}\times \frac{90}{100}\\ & \text{ = Rs}\text{. }76.5\\ & \text{Difference = 100}-76.5\\ & =23.5\\ & \text{Difference}\%\\ & =\frac{23.5}{100}\times 100\\ & =23.5\%\end{array}$$