1.  PLAIN : AUSTERE ::

A. neutral: detached
B. clean : sterile
C. lonely : gloomy
D. deserted : barren

Answer: Option B

Explanation:

(DEGREE OF INTENSITY) Being austere (meaning bare) is a heightened form of being plain; being sterile (meaning free from bacteria or other micro-organisms) is a heightened form of being clean.

The pairs (neutral, detached); (deserted, barren) and (eloquent, talkative) are mere synonyms.(B)