Which of the following is an example of a stay?

A stay is typically a structural support used in various types of equipment, including boilers, to maintain integrity and prevent deformation under pressure. In this context, a rod serves as a stay by providing stability and support to pressure vessels. The design of a boiler might include stays in the form of rods that are installed to help resist the forces exerted by the pressure of the steam or water inside.

Other options like pipe, valve, and flue do not serve the function of providing structural support in the same manner. A pipe serves as a conduit for fluid flow, a valve is a device used for regulating such flow, and a flue refers to the duct that carries exhaust gases away from a boiler or furnace. While each of these components plays a critical role in the operation of a boiler system, none of them specifically acts as a stay, making the rod the appropriate example in this scenario.