The diagram shows a displacement ventilation system. In this system:
Cool supply air (about 65°F) is introduced at floor level at low velocity through floor diffusers.
Warm air rises as it picks up heat from occupants/equipment and stratifies near the ceiling, where it is returned or exhausted.
This creates a stratification layer with cooler, cleaner air in the occupied zone and warmer air above.
This is efficient for comfort, air quality, and energy use in spaces #12 ft high.
Why not other options:
A: This system uses a plenum return, not ducted supply and return for both.
C: It is efficient in summer since it uses warmer supply air and reduced fan energy.
D: Individual comfort control is possible via diffuser location/adjustment.
PDD Reference: ARE 5.0 PDD "Mechanical Systems-Air distribution types," ASHRAE Fundamentals, displacement ventilation diagrams.