The duty of atrue citizen is to thwart social evils as untouchability, dowry system etc.

Solution (By Examveda Team)

Thwart : prevent (someone) from accomplishing something.
Favour : approval, support, or liking for someone or something.
Sustain : strengthen or support physically or mentally.
Promote : support or actively encourage (a cause, venture, etc.); further the progress of.
Frustrate : prevent (a plan or attempted action) from progressing, succeeding, or being fulfilled.

Antonym of Thwart is Promote