Given:
- Cost Price (CP) = Rs. 27.50
- Selling Price (SP) = Rs. 28.60
Step 1: Calculate the gain:
\[
\text{Gain} = \text{SP} - \text{CP}
\]
\[
= 28.60 - 27.50 = 1.10
\]
Step 2: Compute the gain percentage:
\[
\text{Gain Percentage} = \left( \frac{\text{Gain}}{\text{CP}} \right) \times 100
\]
\[
= \left( \frac{1.10}{27.50} \right) \times 100
\]
\[
= 4%
\]
Thus, the correct answer is 4%.