What is the primary purpose of the buoyancy control device (BCD)?

The primary purpose of the buoyancy control device (BCD) is to regulate buoyancy while diving. A BCD allows divers to adjust their buoyancy in the water effectively, enabling them to float at a desired depth, ascend, or descend as needed. By inflating or deflating the BCD, divers can achieve neutral buoyancy, which is crucial for maintaining stability and control during a dive. This ability to control buoyancy enhances safety and comfort, allowing divers to conserve energy and focus on their surroundings and activities without struggling against unwanted floatation or sinking.

The other options do not reflect the primary function of the BCD. Adding extra weight to a diver is not part of its design; instead, a diver's weight system is intended for achieving neutral buoyancy. Measuring depth is typically performed using a depth gauge, not a BCD. While a BCD may carry the air supply in the form of a tank, it does not regulate the air supply itself—this is the role of the regulator.