Which of the following is NOT classified as a plenum space?

A plenum space is an area used for air circulation for heating, ventilation, and air conditioning (HVAC) systems, and is typically a part of the building's air-handling system. It generally includes spaces between the structural ceiling and a drop-down ceiling or raised floor, where air can flow to distribute heating or cooling.

Office space is typically used for work, with individual rooms or open areas, and it does not serve the specific function of air distribution that a plenum space provides. In contrast, other options, such as a mechanical room, cable tray, and electrical closet, can contain portions that are utilized for air circulation or are physically located in areas designated for air movement as part of the building's environmental control systems.

By classifying all the other spaces as plenum areas, they hold functions associated with HVAC technology, handling or directing airflow, which does not apply to typical office spaces. Hence, identifying office space as not being classified as a plenum space aligns with the definition of plenum spaces in the context of the NEC.