Final answer:
The Spanish fort that could be associated with a mission is called a presidio, and Castillo de San Marcos is an example built to defend against imperial challengers.
Step-by-step explanation:
The name for a Spanish fort that may or may not have been attached to a mission is a presidio. An example is the Castillo de San Marcos, which was built to defend St. Augustine against imperial challengers. The Spanish built fortifications like presidios not only for offensive wars but also for defensive strategies, which included protection from rival European powers and privateers. San Francisco's El Presidio is another example of a strategic military fortification established by the Spanish. These structures underscored Spain's military posture during their conquest of the Americas.
To answer the multiple-choice question, the primary reason the Spanish built Castillo de San Marcos was B. to defend against imperial challengers. This stone fortification was constructed to provide a more resilient defense against attacks, such as the devastating raid by Sir Francis Drake in 1586. The strategic location and the presence of cannons were inherently defensive measures essential to protect the Spanish interests and settlements in the New World.