The correct option is
C Credit
'Slur' means an insult or slander.
The correct antonym of the given word is option C, 'credit' which means to publicly acknowledge a contributor's role in an endeavour.
options A, B and D are incorrect because 'promise' means a pledge or a vow, 'hope' means an aspiration or desire for something to happen in the future and 'virtue' refers to a behaviour showing high moral standards.