A synonym is a word that has the same meaning with respect to another word.
The word 'reprisal' means 'an act of retaliation', it refers to a sort of revenge or counterattack. For example, 'Some hostages are believed to have been taken as a reprisal by some soldiers.'
From the options, 'retaliation' means the same: the action of returning a military attack or counterattack. Consider: Some hostages are believed to have been taken in retaliation by some soldiers. Hence it is the synonym here and option D is correct.
'Denial' means 'to refuse to accept something'. (for example: She lay there in denial.)
'Reluctance' means 'unwillingness to do something'. (for example: He was reluctant towards surrendering himself.)
'Unequivocal' means 'leaving no doubt', not ambiguous, clear. (an unequivocal reply)
These words do not mean the same as the given word, hence options A, B and C are incorrect.