Overextension of the fly section of an extension ladder is prevented by __________.

The correct answer is that stops are used to prevent overextension of the fly section of an extension ladder. Stops are designed to limit the travel distance of the fly section, ensuring that it cannot be extended beyond a certain point. This is crucial for safety, as overextending the ladder can lead to instability and increase the risk of accidents.

Additionally, while other components like locks, braces, and guides serve important functions in the operation and stability of ladders, they do not specifically address the issue of preventing overextension. Locks ensure that the sections remain secure when extended, braces provide structural support, and guides help to align the sections during operation, but it is the stops that directly prevent the fly section from being extended too far. Understanding the specific role of stops is essential for safely using extension ladders in firefighting and other scenarios.