What type of protective device is made with a double tree design and includes a solid end?

The correct choice is combat arms earplugs. These earplugs are specifically designed for use in noisy environments, such as combat situations, where protection from loud sounds is essential. The double tree design allows for flexibility in sound attenuation; users can choose to block out all noise or allow certain sounds to be heard. The solid end component provides maximum noise reduction when complete silence is needed, which is critical in field operations where situational awareness can be crucial.

In contrast, standard ear defenders do not have this dual-functionality and are primarily designed for general noise reduction in various environments, without the specific features that combat arms earplugs provide. Noise-cancelling headphones are focused more on canceling ambient noise for a quiet listening experience rather than providing varying levels of protection from loud, damaging sounds. Disposable earplugs, while similar in their purpose of noise reduction, lack the advanced design and functionality that make combat arms earplugs suitable for tactical use.