Which of the following aircraft is the largest in terms of cargo capacity?

The C-5 Galaxy is renowned for its impressive cargo capacity, making it the largest military transport aircraft in terms of volume. It can carry an extensive range of oversized cargo, including tanks, helicopters, and large equipment. The C-5 Galaxy can accommodate a maximum payload of approximately 140 tons and features a unique cargo hold with the ability to load and unload from both the front and the rear of the aircraft. This versatility allows it to transport a wide variety of military vehicles and heavy cargo that are vital for operational deployment.

While the C-17 Globemaster III also has a significant cargo capacity and plays a crucial role in rapid strategic airlift of troops and cargo, it is still smaller than the C-5. The C-130 Hercules, while a highly capable tactical airlifter, is designed for smaller cargo loads and operational flexibility in challenging environments, making it less capable in terms of sheer capacity compared to the C-5. The KC-135 Stratotanker is primarily a refueling aircraft and, consequently, does not focus on cargo transport in the same way that the other aircraft do.

This context highlights the C-5 Galaxy's dominance in cargo capacity compared to the other aircraft listed, firmly establishing it as the correct answer.