Which best describes an antagonist?

the main character, often a hero

a well-developed character who is important to the story

the character who is in conflict with the protagonist, a “bad guy”

a poorly developed character

Verified Answer :

The third answer, "the character who is in conflict with the protagonist, a "bad guy"".  This is because that is what antagonist means, except that he/she isn't always a "bad guy".

The character in conflict with the protagonist, but in the best stories he/she is also well developed and important to the plot.