Step 1: Definition of "flagrant."
"Flagrant" describes actions or behavior that are conspicuously offensive, blatant, or openly wrong and disgraceful.
Step 2: Evaluation of options.
- "Notorious" signifies widespread negative recognition, which shares a connection with "flagrant" through public awareness but is not its opposite.
- "Blatant" means obvious and openly done, making it nearly synonymous with "flagrant."
- "Unobtrusive" denotes something that does not attract attention, thus representing the opposite of "flagrant," which implies being noticeable.
- "Adjacent" refers to proximity and is irrelevant to the meaning of "flagrant."
Step 3: Determination of the antonym. "Unobtrusive" is the appropriate antonym because it signifies discretion and a lack of prominence, contrasting with "flagrant," which implies conspicuousness.Therefore, the correct answer is 3. Unobtrusive.
Final Answer: \[\boxed{\text{The correct answer is 3. Unobtrusive.}}\]