Explanation:

To find the simple interest earned on Rs 6,000 in 21 months at an 8% annual rate, we can use the simple interest formula:

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

- Principal (P): Rs 6,000

- Rate (R): 8% per annum

- Time (T): 21 months, which is equivalent to 21/12 years or 1.75 years

Plug these values into the formula:

$$ \text{Simple Interest} = \frac{6000 \times 8 \times 1.75}{100} = 840 $$

- Option: 1 Rs 750

- Option: 2 Rs 880

- Option: 3 Rs 620

- Option: 4 Rs 840