Overextension of the fly section of an extension ladder is prevented by:

The correct choice is linked to the structural features of an extension ladder designed to enhance safety during operation. Stops are specifically engineered components that limit how far the fly section of an extension ladder can extend. By preventing overextension, these stops help ensure that the ladder maintains its stability and structural integrity when in use.

When the fly section reaches its maximum extension, the stops engage and halt any further movement, providing an essential safety mechanism that protects the ladder and the firefighter using it. This is crucial because overextending the fly section can lead to balance issues or even a risk of structural failure, increasing the hazard for the firefighter on the ladder.

In contrast, guides, pawls, and locks serve different purposes within the ladder system. Guides help align the fly section as it is extended but do not restrict the extension distance. Pawls are mechanisms that engage to lock the sections in place but are not primarily designed to prevent overextension. Locks work to secure the sections in their extended position but are less focused on limiting extension distance during deployment. Thus, stops are the key feature specifically intended for controlling the maximum reach of the fly section.