What stake has a long round end and a smaller flat end for riveting and forming on round objects?

The correct choice describes a hollow mandrel, which specifically features a long round end and a smaller flat end. This design is particularly useful in HVAC sheet metal work because the round end can be used to form and shape ductwork and other rounded metal components without causing damage to the material. The flat end is used for riveting, allowing for secure fastening of sheets or components together.

In contrast to other options, a bench plate serves as a work surface for metalworking tasks but does not have the specific shaping features needed for riveting and forming. A solid mandrel, while also useful in forming, lacks the hollow interior that can accommodate various applications, particularly in working with different sizes of materials. Seaming refers to a technique rather than a tool or stake and does not provide the specific functional benefits associated with the hollow mandrel in the context of forming and riveting.