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

The correct answer is D. frustrate.

A true citizen should frustrate social evils such as untouchability and dowry system. This means that they should work to stop these practices from happening and to make sure that everyone is treated equally.

Option A, favor, is incorrect because a true citizen should not support social evils. Option B, sustain, is incorrect because a true citizen should not help to keep social evils going. Option C, promote, is incorrect because a true citizen should not encourage social evils.

It is important for citizens to stand up against social evils and to work to make their communities more just and equitable.