Which is the best definition of protagonist?

a.
the character in a work of fiction who tries to stop the main character from achieving his or her specific purpose

b.
the character in a work of fiction who tries to achieve a particular purpose and who is usually the main character

c.
a literary device that authors use in order to force action in a story rather than letting events unfold naturally

d.
a character whose personality and attitude clash with the personality and attitude of the main character