Correct Answer - Option 4 : 32%

**Given**

Amount = 9 principal

Time = 25 years

**Formula used**

If a sum of money becomes x times in t years at simple interest, then formula for calculating rate of interest is as follows:

Rate = [100(x - 1)/t]%

**Calculation**

Time = 25

x = 9 times

R = [100(9 - 1)/25]%

⇒ R = (100 × 8/25)%

⇒ R = 32%

**∴ Rate of interest is 32%**