The sentence describes connoisseurs, who are experts or enthusiasts with deep knowledge in a specific area (e.g., wine, art). We need an adjective that describes the type of knowledge they gather on a single subject.

Key points:
1. Connoisseurs are known for their extensive, detailed, or specialized knowledge.
2. The blank should reflect the quantity or quality of their knowledge.

Analyzing the Options
A. scant
- Meaning: Very little or insufficient.
- Fit: Contradicts the idea of connoisseurs as knowledgeable.
- Reject.
B. copious
- Meaning: Abundant or extensive in quantity.
- Fit: Connoisseurs gather a lot of knowledge. This works.
- Strong candidate.
C. spurious
- Meaning: False or fake.
- Fit: Connoisseurs seek authentic knowledge, not fake information.
- Reject.

D. peculiar
- Meaning: Strange or unusual.
- Fit: Doesn't describe the amount or nature of knowledge.
- Reject.
E. arcane
- Meaning: Secret, obscure, or known only by experts.
- Fit: Connoisseurs often delve into specialized, esoteric knowledge. This works.
- Strong candidate.

Comparing "Copious" vs. "Arcane"
- Copious: Focuses on the amount of knowledge (connoisseurs gather a lot).
- Arcane: Focuses on the nature of knowledge (specialized, expert-level).

Both fit, but the sentence emphasizes gathering knowledge, which leans slightly toward quantity ("copious"). However, "arcane" is also valid if the focus is on depth/specialization.

Contextual Clues
The phrase "gather $\qquad$ knowledge on a single subject" suggests:
- "Copious" aligns with the act of gathering (accumulating a large amount).
- "Arcane" aligns with the result (knowledge becomes specialized).

Since the verb "gather" emphasizes accumulation, "copious" is the more precise choice.

Why Other Options Fail
- Scant/spurious: Contradict connoisseurs' expertise.
- Peculiar: Irrelevant to knowledge accumulation.

Final Answer
B. copious is the best choice, as connoisseurs gather abundant knowledge.

Correct Answer: B

Note: "Arcane" (E) is plausible but less direct for the verb "gather." The primary answer is B. copious.