Explanation:

- We start with an initial principal of Rs.8,500.

- In 6 years, it grows to Rs.11,050 using simple interest.

- To find the rate of interest, use the formula:

\[ \text{Simple Interest (SI)} = \text{Final Amount} - \text{Principal} = Rs.11,050 - Rs.8,500 = Rs.2,550 \]

- Calculate the rate:

\[ \text{SI} = \frac{\text{Principal} \times \text{Rate} \times \text{Time}}{100} \]

\[ \frac{8,500 \times \text{Rate} \times 6}{100} = 2,550 \]

\[ \text{Rate} = \frac{2,550 \times 100}{8,500 \times 6} = 5\% \]

- New rate if increased by 1.8 times:

\[ \text{New Rate} = 5\% \times 1.8 = 9\% \]

- Calculate final amount for the same principal in 5 years at the new rate:

\[ \text{SI} = \frac{8,500 \times 9 \times 5}{100} = Rs.3,825 \]

\[ \text{Final Amount} = Rs.8,500 + Rs.3,825 = Rs.12,325 \]

- Option 4: Rs.12,325 is the correct answer.