The authorities inflicted harsh penalty on him for indulging in forgery.

Answer: Option D

Penalty : a punishment imposed for breaking a law, rule, or contract.
Commendation : formal or official praise.
Default : failure to fulfil an obligation, especially to repay a loan or appear in a law court.
Concession : a thing that is granted, especially in response to demands.
Reward : a thing given in recognition of service, effort, or achievement.

Antonym of Penalty is Reward