Which of the following is NOT considered a confined space?

A confined space is typically defined as an area that is not intended for continuous human occupancy and has limited means for entry or exit, making it potentially hazardous. The correct choice is the MCC (Motor Control Center) room, as it generally provides space for personnel to move about safely and is designed with accessibility in mind for maintenance and operational tasks.

In contrast, deaerators, boilers, and stacks can present specific risks associated with confined spaces. Deaerators often have limited access and can contain hazardous atmospheres, making them confined spaces. Boilers can also present similar hazards, as they are often enclosed and have confined access points. Stacks, which are vertical structures used to vent flue gases from combustion processes, may have limited access and pose hazards associated with working at heights or within enclosed spaces.

Therefore, the MCC room represents a space that is typically safe for personnel engagement and maintenance activities, distinguishing it from the other options, which are associated with greater confinement and potential hazards.