The correct answer is B) Impeller. In a gas turbine engine lube oil system, the air and oil are primarily separated in the air/oil separator through the use of an impeller. The impeller spins rapidly, creating a centrifugal force that separates the lighter air from the heavier oil, allowing the oil to be recirculated back into the system while the air is vented out. The other options are incorrect because a filter, heater, or strainer are not the primary means of separating the air and oil in this type of system. These components may be present in the system, but they do not perform the primary air/oil separation function.