The correct answer is B.
The conflict presented in "To Build a Fire," is between man and nature.
The man had been warned that it was too cold to travel, but being arrogant and smug, he decided to travel nonetheless. While was walking, the man fell through a thin patch of ice and could not start a fire to warm himself and save his own life.
This story shows the eternal conflict in which men underestimates nature and its dangers and believes he can conquer it all.