In a bent-tube watertube boiler, the furnace walls are formed by:

In a bent-tube watertube boiler, the correct answer identifies that the furnace walls are formed by waterwalls. Waterwalls are constructed from a series of tubes that are filled with water, serving both structural and functional purposes. These walls are positioned within the furnace, where they not only help contain the combustion process but also absorb heat from the products of combustion.

The key characteristic of waterwalls is that they allow for efficient heat transfer and help keep the structural integrity of the boiler by managing the high-temperature environment within the furnace. By utilizing water as a coolant, they effectively reduce the chances of overheating and damage to the boiler components.

While other options like furnace walls, fireside walls, and waterside walls may refer to various parts of a boiler, they do not specifically represent the arrangement and function of the tubing in a bent-tube watertube boiler as accurately as waterwalls do. Thus, the composition of the furnace walls being primarily water walls highlights their crucial role in both the operation and safety of the boiler system.