11 oranges are bought for Rs. 10 and 10 oranges are sold for Rs. 11. Find the gain (or) loss percentage.

The correct option is C: $21\%$ gain

Consider the price of each orange

CP of 1 orange $=Rs.\frac{10}{11}$, SP of 1 orange $=Rs.\frac{11}{10}$

$\frac{10}{11}=0.90$ and $\frac{11}{10}=1.1$

$SP>CP,$ then there is gain.

$Gain\%=\frac{SP-CP}{CP}\times 100\%$

$=\frac{\frac{11}{10}-\frac{10}{11}}{\frac{10}{11}}\times 100\%$

$=\frac{(11\times 11)-(10\times 10)}{10\times 11}\times \frac{11}{10}\times 100\%$

$=\frac{121-100}{100}\times 100\%$

$=21\%$

Since the profit percentage is positive, the net transaction is a profit.

Hence, the gain $\%$ is $21\%$