Explanation:

Correct Option is 4 - Rs 26,000

Solution

Total interest after 18 years = Rs. 35,100
Formula used:
S.I = (P x R x T)/100
Calculation:
Let the principal be 'P'.
For the first 8 years
S.I = (P x 7.25 x 8)/100
⇒ S.I = 58P/100
For the next 6 years
S.I = (P x 8.5 x 6)/100
⇒ S.I = 51P/100
For the next 4 years
S.I = (P x 6.5 x 4)/100
⇒ S.I = 26P/100
Total Interest in 18 years = Rs. 35,100
(58P/100) + (51P/100) + (26P/100) = 35100
⇒ 135P/100= 35100
⇒ P = 26000
:. The amount invested is Rs. 26,000.