Which aircraft is designed for transport of cargo?

The C-26 Metro III is specifically designed for the transport of cargo, making it the correct choice for this question. It is a twin-engine turboprop aircraft that can accommodate various types of cargo, including passengers and freight, depending on mission requirements. The C-26 has a larger cabin space and a design that supports transporting supplies, equipment, and personnel, which reflects its primary role as a cargo transport aircraft in military and civilian operations.

In contrast, the other options are not primarily designed for cargo transport. Trainer aircraft are focused on pilot training and do not typically have the capacity or configuration for cargo. The C-21 Learjet, while capable of some cargo transport, primarily serves as a passenger transport aircraft for military officials and VIPs. The C-37A Gulfstream V is similarly used for transporting high-ranking officials and is designed for executive transport rather than efficient cargo handling. Therefore, the C-26 Metro III stands out as the dedicated cargo transport option in this list.