The correct option is C reward
The word 'penalty' refers to a punishment imposed for breaking a law or rule.
Option A, commendation refers to a formal or official praise.
Option B, default refers to failing to fulfill an obligation.
Option C, concession refers to a thing that is granted especially in response to demands.
Option D, reward refers to a thing given recognition of service, effort or achievement.
Thus, according to the meaning, the word 'reward' is the opposite of the word 'penalty'.