This is a Sentence Equivalence question, and you need to choose two words that are synonyms and logically complete the sentence.

Breakdown of the Blank
The sentence is about what makes a trivia game successful: it must be mindful of the players' knowledge. The consequence of failing this awareness is that players "most quickly lose interest."

Therefore, the blank must describe a topic that causes players to lose interest because it is outside their general "range of knowledge." We need words that mean too difficult, too specialized, or too little known.
- A. eclectic: Composed of elements drawn from various sources. This suggests variety, not difficulty, so it doesn't explain why people would lose interest.
- B. obscure: Not discovered or known about; uncertain. This is a perfect fit. If a topic is too little known, players can't answer and lose interest.
- C. recondite: Dealing with a very profound, difficult, or obscure subject. This is a strong synonym for "obscure" and fits the idea of a topic being outside the common range of knowledge.
- D. ephemeral: Lasting for a very short time. This describes duration, not difficulty.
- E. prosaic: Having the style or diction of prose; dull. While dullness could cause loss of interest, the sentence implies the problem is a lack of knowledge (too niche), not a lack of excitement.
- F. topical: Relating to the subjects of current interest. This is the opposite of a topic that is too difficult or unknown.

The two words that mean too specialized or unknown and thus cause people to lose interest are obscure and recondite.

The correct choices are B. obscure and C. recondite.