Return air shall not be taken from a room with a volume less than ______ percent of the entire volume served by the system.

The correct choice reflects a widely accepted guideline in HVAC (heating, ventilation, and air conditioning) systems that emphasizes the importance of air quality and circulation. Specifically, the rule indicates that return air should not be drawn from rooms that are too small in relation to the overall space being served by the system. Maintaining this minimum volume helps ensure adequate fresh air intake and prevents the negative effects of high recirculation rates from smaller areas, which can lead to stale air and potential buildup of contaminants.

By establishing a threshold of 25 percent for return air from relatively small rooms, the system promotes better air mixing and quality throughout the larger space. This principle often stems from regulatory codes or industry standards to sustain effective ventilation practices. A lower percentage might compromise air quality, while aiming for volumes significantly higher than 25 percent may be impractical or resource-intensive.