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Explanation:

Let the interest accrued during first year = I and rate of interest be r%

We know in case of compound interest, interest for each year increases by r% every year.

∴ Interest for 2^nd year = I × $\left(1+\frac{r}{100}\right)$ = 806.25   …(1)
Interest for 3^rd year = I × ${\left(1+\frac{r}{100}\right)}^2$ = 866.72   …(2)

(2) ÷ (1)

⇒ $\left(1+\frac{r}{100}\right)$ = $\frac{866.72}{806.25}$ = 1 - $\frac{60.47}{806.25}$

⇒ r = 7.5%

∴ Interest for 4^th year = Interest for 3^rd year × (1+r/100)

= 866.72 × $\left(1+\frac{7.5}{100}\right)$ = 931.72

Hence, option (c).