Option 1 states that a man requires 1000 cubic feet of air every 20 minutes. This means that for every 20 minutes, a man needs to have 1000 cubic feet of fresh air.

Option 2 is incorrect, as it states that a man requires 1000 cubic feet of air every 20 seconds. This would mean that a man would need a much larger quantity of air in a shorter timeframe, which is not feasible.

Option 3 is also incorrect, as it states that a man needs 1000 cubic feet of air every 10 minutes. This would mean that a man would require air more frequently than option 1 suggests.

Option 4 is incorrect as well because it states that a man requires 1000 cubic feet of air every 10 seconds. This would suggest an extremely high rate of air consumption, which is unrealistic.

Therefore, the correct answer is option 1, stating that a man requires 1000 cubic feet of air every 20 minutes.