The correct option is B concord
The word 'discord' refers to disagreement between people.
Option A, tranquility refers to the state of being calm.
Option B, familiarity refers to relaxed friendliness or intimacy between people.
Option C, concord refers to agreement or harmony between people.
Option D, benevolence refers to the quality of being well menaing.
Thus, according to the meaning, the word 'concord' is the opposite of the word 'discord.'.