Let's break down the sentence to understand the context and the meaning required for the blank:
- "The fastidious guest was so $\qquad$ while dining that it diminished the hostess' estimation of her."
- "Fastidious": This is a key clue. It means very attentive to and concerned about accuracy and detail; hard to please; meticulous. This implies a high standard or pickiness.
- "diminished the hostess' estimation of her": This is the result of the guest's behavior. The behavior must have been negative or annoying to the hostess.
- The blank needs a word that describes a negative behavior during dining that stems from being "fastidious" and would annoy a hostess.

Let's evaluate the given options:
- A. finicky: Fussy about food, clothes, or other matters; difficult to please. This aligns perfectly with "fastidious" and describes a behavior (fussiness while dining) that would certainly diminish a hostess's estimation.
- B. boorish: Rough and bad-mannered; rude. While being rude would diminish estimation, "boorish" doesn't directly connect to "fastidious" or dining fussiness. A fastidious person might be rude, but their rudeness isn't usually because they are fastidious in the way fussiness is.
- C. loquacious: Tending to talk a great deal; talkative. This describes talking a lot, which might be annoying but isn't directly related to being "fastidious" or dining behavior in a way that suggests fussiness.
- D. stolid: Calm, dependable, and showing little emotion or animation. This is a neutral or even positive trait, and it wouldn't typically diminish someone's estimation in this context.
- E. curt: Abrupt or brief in speech or manner; rude. Similar to "boorish," this describes rudeness, but not behavior stemming from being "fastidious" while dining.

The word "fastidious" strongly suggests pickiness or fussiness. When applied to dining, this leads directly to "finicky." Such behavior would understandably annoy a hostess and diminish her estimation.

The final answer is
A