What feature of air tank drains in an air brake system differentiates the manual and automatic methods of water and oil expulsion?

Answer: D — The mode of operation for expelling water and oil

In an air brake system, the feature that differentiates the manual and automatic methods of water and oil expulsion in air tank drains is the mode of operation for expelling water and oil. Manual air tank drains require the driver or maintenance personnel to physically open a valve or mechanism to release any accumulated water or oil from the system. On the other hand, automatic air tank drains are designed to expel water and oil without manual intervention. This difference in operation is crucial for the efficient maintenance and performance of the air brake system. Options A, B, and C are not directly related to the distinction between manual and automatic air tank drains for expelling water and oil in an air brake system.