Explanation:

To determine how many persons are needed, let's break down the problem:

- We are given that 18 persons working 8 hours/day can complete 3 units in 10 days.

- Let's calculate the total man-hours required for 3 units:

- 18 persons × 8 hours/day × 10 days = 1440 man-hours.

- Thus, 1440 man-hours for 3 units means 1 unit = 480 man-hours.

- For 5 units, we need 5 × 480 = 2400 man-hours.

- New scenario: Complete 5 units in 16 days, working 6 hours/day.

- Calculate the total available hours for each person: 16 days × 6 hours/day = 96 hours/person.

- Total persons required = Total man-hours needed / Total available hours per person = 2400 / 96 = 25.

- Correct answer: Option 1 - 25 persons