Which of the following sentences most clearly describes a character
experiencing internal conflict?
O A. Nina wants to win the karate championship, but her opponent is
extremely talented.
O B. Angela wants to become a vampire, but Vladimir refuses to bite
her.
O C. Min wants to fight in Vietnam, but the army says he is too young to
enlist.
O D. Morris wants to ask a girl out, but he can't imagine anyone liking
him.

Conflict can be defined as a struggle between two opposing forces. In literature, common types of conflict are character vs self, character vs society, or character vs nature, among others. Conflicts can be external or internal.

External conflicts happen between the character and an outside force - another person, society, nature, financial difficulties etc. Internal conflicts take place in the character's mind, usually because of the character's self - insecurity, difficulty to change one's mind, difficulty to adapt to a new situation, stress, fear, anger etc.

Having that explanation in mind, it is easy to see that options A - C represent external conflicts caused by outside forces. Option D, however, describes an internal conflict, caused by the character's insecurity about himself.