Which word would you associate with the concept of a protective stronghold?

The term "citadel" is associated with a protective stronghold because it specifically refers to a fortress or stronghold that is typically situated on high ground and is designed to protect a city or area from invasion. Historically, citadels served as a refuge during times of conflict, providing not just physical protection but also serving as a symbol of defense and resilience.

In contrast, the other options do not relate to the concept of a protective stronghold. "Cache" refers to a store of goods or valuables, often hidden or stored away for future use, without any connotation of defense. "Capacious" describes something that is spacious or able to hold a large amount, focusing on capacity rather than protection. "Caption" refers to a title or explanation accompanying an illustration or photograph, which is unrelated to any notion of physical security or fortification. Thus, "citadel" is the most fitting word for the concept of a protective stronghold.