Antonyms are opposites of something.
Ex : opposite of good is bad.
Synonyms are the another word for the original one.
Ex: Concerned means worried or nervous
Acronyms means a abbreviation for something instead of lengthy sentence.
Ex: OMG = Oh my God.
The correct answer is "I can't believe he has <em>swum </em>to the island." Swum is the past principle of "swim." However, if you drop the word <em>has </em>the answer would be D and the sentence would appear much more grammatically appealing. :)