How do you salute on a challenging post when armed with the service rifle?

When saluting on a challenging post while armed with a service rifle, the correct procedure is to perform a "Present Arms." This action is a formal and respectful way to acknowledge an officer or the national flag while carrying a weapon. Presenting arms involves holding the rifle at the proper position in front of your body, typically at an angle aligned with the centerline, which demonstrates both discipline and respect. This is particularly important in a challenging post scenario where the presence of arms emphasizes the solemnity and ceremonial nature of the duty.

The other choices represent different forms of salutation or positions with a weapon that are not appropriate for this specific context. For instance, while a hand salute is a common gesture of respect, it is not the protocol when a service member is bearing arms. Additionally, "Eyes Right" is typically used during marching or parading, and "Right Shoulder Arms" is a position of carry that does not involve saluting but rather prepares the rifle for movement. Therefore, "Present Arms" is the only choice that aligns with the protocols for saluting while armed on a challenging post.