Explanation

(c)
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Although thre number of –OH groups is increasing in $H_{3}PO_{2}$ ( 1 OH group), $H_{3}PO_{3}$ (2 OH group) and $H_{3}PO_{4}$ (3 OH group), yet acidity does not increase much. This is due to the fact that the number of unprotonated oxygen, responsible for enhancement of acidity due to inductive effect, remains the same, as a result dissociation constant also remains nearly same.