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:

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

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

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

- New rate if increased by 1.8 times:

$$\text{New Rate}=5\mathrm{\%}\times 1.8=9\mathrm{\%}$$

- 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.