A man buys 15 mangoes for a rupee. How many mangoes had he sold for a rupee so that there is a loss of 25 percent?

The man buys 15 mangoes for 1 rupee.

He incurs a loss of 25% when selling the mangoes.

Since he buys 15 mangoes for 1 rupee, the cost price per mango is:

​$$\text{CP per mango} = \frac{1}{15}$$ 

With a 25% loss: 

 = $$\text{SP per mango}=\frac{1}{15} \times \frac{75}{100}\\ \ \\ \frac{1}{20}\ Rs$$ 

 $$\text{Number of mangoes per rupee =}\frac{1}{\text{SP per mango}} = \frac{1}{\frac{1}{20}}\\ \ \\ = 20 \ \text{mangoes}$$ 

Thus, correct option is (b).