Explanation:

Let's break down the options and explain the scenario:

- A can do the work in 3 hours, so A's rate = 1/3 per hour.

- B in 6 hours: B's rate = 1/6 per hour.

- C in 9 hours: C's rate = 1/9 per hour.

- D in 12 hours: D's rate = 1/12 per hour.

- Only one person at a time, and not in consecutive hours.

- We want the minimum hours to finish the work.

Optimal way:

- Alternate the two fastest workers: A and B, as using A for 1 hour, then B for 1 hour, then again A, and so on.

- For 2 hours: Work done = (1/3 + 1/6) = 1/2.

- For 4 hours (two cycles): Work done = 2 × (1/3 + 1/6) = 1 whole work.

So, minimum number of hours is 4.

Options Analysis:

- Option 1 (36/25) ˜ 1.44 hours: Impossible as nobody can finish that fast given constraints.

- Option 2 (12/5) = 2.4 hours: Again not possible under the rules.

- Option 3 (4): Matches our calculation.

- Option 4 (2): Too short, not enough work can be done.

Correct Answer: Option 3 (4 hours)