The correct answer is C) self-contained breathing apparatus. Vessels carrying cargo must have a self-contained breathing apparatus (SCBA) as part of the fireman's outfit. This is required by regulations to provide the fireman with a reliable source of clean, breathable air when entering hazardous environments, such as cargo holds that may contain toxic or oxygen-deficient atmospheres. The other options are incorrect because: A) a canister-type gas mask would not provide a sufficient supply of air, B) a fresh-air breathing apparatus relies on an external air supply, and D) a combustible gas indicator is a separate piece of equipment used to detect the presence of flammable gases, but does not provide respiratory protection.