When is it appropriate to wear a cover indoors?

Wearing a cover indoors is appropriate when a Marine is under arms and on duty, as this is in line with military protocol and professionalism. When a Marine is performing official duties, especially in a capacity that involves the use of their weapons, wearing the cover signifies readiness and maintains the military's standard of uniformity.

In other circumstances, such as in a classroom or during formal events, proper military etiquette typically dictates that covers are to be removed. This practice shows respect for the indoor setting and acknowledges specific customs associated with various formal or educational environments. Being outside is also not a context that allows for wearing covers indoors, as it does not align with the behavior expected in non-indoors situations. Thus, the correct answer highlights the specific context of duty and professionalism associated with military conduct.