If there is no _____ movement within a room, the air will tend to stratify.

The correct answer is air. In the context of HVAC systems and indoor environments, effective air movement is critical to maintaining a uniform temperature and preventing stratification, which occurs when warmer air rises and cooler air settles near the floor. When air is stagnant, it can lead to layers of differing temperatures, with warmer air accumulating at the ceiling and cooler air lingering at ground level. This phenomenon can create discomfort for occupants and result in inefficient heating or cooling.

In a well-designed HVAC system, fans and other mechanisms are employed to ensure that air circulates throughout the space, promoting even distribution of temperature and minimizing the risk of stratification. By facilitating air movement, the system enhances thermal comfort and operational efficiency, ensuring that all areas of the room receive adequate heating or cooling.

The other options, such as heat, humidity, and light, play important roles in indoor environmental quality, but they do not directly relate to the issue of air movement leading to stratification. Stratification is specifically tied to the circulation of air, making it the key factor in this scenario.