Explanation:

- The simple interest on the principal for 4 years at 10% p.a. is Rs 20,000.

- Using the simple interest formula, \( \text{Principal} = \frac{\text{Interest}}{(\text{Rate} \times \text{Time})} = \frac{20000}{0.10 \times 4} = Rs 50,000 \).

- Now, calculate the compound interest on Rs 50,000 for 2 years at 5% p.a.

- Use the formula \( \text{Compound Interest} = \text{Principal} \times \left(1 + \frac{\text{Rate}}{100}\right)^n - \text{Principal} \).

- For Rs 50,000 at 5% for 2 years:

\( \text{CI} = 50000 \times \left(1 + \frac{5}{100}\right)^2 - 50000 \).

\( \text{CI} = 50000 \times 1.1025 - 50000 = Rs 5,125. \)

- Option 1: Rs 5,125 is the correct answer.