When is wearing of a hat in a building permissible?

Wearing a hat indoors while under arms is permissible as it is part of military customs and courtesies. When service members are in uniform and have their headgear on, it signifies that they are in an alert and ready status. This practice is rooted in military tradition, which emphasizes the importance of being respectful when in uniform and under arms.

In many cases, wearing headgear indoors is discouraged to maintain decorum; however, an exception is made when personnel are required to remain in a ready state, symbolizing their commitment to duty and readiness for immediate action. The other scenarios involving ceremonial events, military parades, and designated areas may have their own rules regarding dress, but the key point for this correct answer is the specific situation of being under arms, where headgear is appropriate regardless of the indoor setting.