Option 1 -> Bitter is actually a synonym of acrimonious, not an opposite.
Option 2 -> Resolute means determined or firm, which is unrelated to the harsh/angry nature of acrimonious.
Option 3 -> Vague means unclear or indefinite, which doesn't relate to the bitter tone of acrimonious.
Option 4 -> Amicable means friendly and peaceable, the direct opposite of harsh and bitter.
Hence, Option 4: Amicable -> Acrimonious describes bitter, harsh, or angry interactions, while amicable describes friendly, pleasant, and harmonious ones, making them perfect antonyms -> correct