Explanation:

- First, let's determine the number of officers in each class based on their ratio.

- The ratio given is 5:4:1 for class-A, B, and C.

- This means out of every 10 officers, 5 are class-A, 4 are class-B, and 1 is class-C.

- Since there are 40 officers in total:

- Class-A: \( \frac{5}{10} \times 40 = 20 \) officers

- Class-B: \( \frac{4}{10} \times 40 = 16 \) officers

- Class-C: \( \frac{1}{10} \times 40 = 4 \) officers

- Calculate the total daily salary for each class:

- Class-A: \( 20 \times 600 = 12,000 \)

- Class-B: \( 16 \times 750 = 12,000 \)

- Class-C: \( 4 \times 1,000 = 4,000 \)

- Total daily salary for all officers: \( 12,000 + 12,000 + 4,000 = 28,000 \)

- Total monthly salary (30 days): \( 28,000 \times 30 = 840,000 \)

- Monthly average salary: \( \frac{840,000}{40} = 21,000 \)

- Thus, the correct option is:

Option 2: 21,000