The relationship between STUBBORN and ADAPTABLE is antonymy — opposites in meaning.
Option (A) — stupid : bright — is also antonymy (dull vs intelligent).
Option (C) — inherent : extraneous — is antonymy (innate vs external).
Option (D) — friend : enemy — is antonymy (ally vs opponent).
Option (B) — moral : amoral — is different; “amoral” means lacking moral sense or being indifferent to right and wrong, not necessarily the direct opposite of “moral” in the way “immoral” would be. This makes it a weaker or mismatched antonym relationship compared to the others.