Which part of the upper receiver assembly protects the interior from foreign matter when not in use?

The correct response is the ejection port cover. This component serves an essential role in protecting the interior of the upper receiver assembly from dirt, dust, moisture, and other foreign substances that could impede the rifle's functioning when it is not in use. The ejection port cover closes the ejection port, which is exposed during operation, thus safeguarding the internal mechanisms of the firearm. Having this protection is particularly crucial in environments where debris is prevalent, enhancing the reliability and longevity of the rifle.

The ejection port itself, while critical for expelling spent cartridges, does not offer the same protective function when the firearm is idle. The forward assist assembly is primarily designed to help ensure that the bolt fully closes when chambering a round, rather than offering any degree of environmental protection. The hand guard provides a grip surface and assists in heat insulation but does not cover the ejection port or contribute to the overall defense against foreign matter. Therefore, the ejection port cover is recognized as the specific part that serves to prevent contamination when the rifle is not operational.