Explanation:

- 30 men dig a pond in 25 days. The work can be represented as the product of men and days.

- This means: $30\text{ men}\times 25\text{ days}=750\text{ man-days}$.

- The same pond needs to be dug in 15 days.

- To find the number of men needed, divide the total man-days by 15 days.

- Calculation: $750\text{ man-days}÷15\text{ days}=50\text{ men}$.

- Option 3, 50 is correct.