1) Explanation

Let's break down the sentence to understand the context and the meaning required for the blank:
- "Known for their devotion to their masters, dogs were often used as symbols of
$\qquad$ in Medieval and Renaissance paintings."
- The crucial clue is "devotion to their masters." This describes a core characteristic of dogs.
- The blank needs a word that is synonymous with "devotion" or expresses a very similar concept, as dogs are used as symbols of this quality.

Let's evaluate the given options:
- A. treachery: Betrayal of trust; deceptive action. This is the opposite of "devotion."
- B. opulence: Great wealth or luxuriousness. This is irrelevant to "devotion."
- C. allegiance: Loyalty or commitment of a subordinate to a superior or of an individual to a group or cause. This is a very strong synonym for "devotion" and fits perfectly as something dogs embody towards their masters.
- D. fidelity: Faithfulness to a person, cause, or belief, demonstrated by continuing loyalty and support. This is also a very strong synonym for "devotion" and "allegiance," and it's a quality famously associated with dogs.
- E. antiquity: The ancient past, especially the period before the Middle Ages. This refers to a time period, not a quality of devotion.
- F. valor: Great courage, especially in battle. While dogs can be valorous, the sentence specifically highlights their "devotion to their masters," making loyalty-related terms a better fit than courage.

Conclusion:

The phrase "devotion to their masters" is the direct clue. Both "allegiance" and "fidelity" are strong synonyms for devotion, loyalty, and faithfulness.

The final answer is $\(\mathrm{C}, \mathrm{D}\)$.