Part 1:
D. quick  "Quick temper" is the correct collocation.
D. formidable  Formidable means causing fear or respect through being impressively powerful, intense, or capable. In this context, it fits best as it suggests a significant change.
C. undermined  Undermined means to damage or weaken something, especially gradually or insidiously. This fits the context of confidence being negatively affected.
D. wasn't he  This is a tag question. "Was" (positive) changes to "wasn't" (negative) and "he" remains "he".
C. at all  "At all" is used to emphasize the lack of importance in this context.
A. well aware  "Well aware" means to be fully conscious or knowledgeable about something.
Part 2:
WIDTH (The noun form of wide is width)
DISAGREEMENT (Add "dis" as a prefix to make it negative and "ment" to form a noun)
POPULARITY (The noun form of popular is popularity)
FORTUNATELY (Add "ly" to make it an adverb)
INHABITANTS (The noun form referring to people who live in a place is inhabitants)