Correct Answer :
D. Bright : Resplendent
Stentorian refers to very loud, hence the relationship in the given words is one of degree. This relationship of degree can be seen in option (d) as resplendent can refer to being very bright or shiny.
Option (a) is wrong, because mild is a general term, and isnt especially talking about mild voice or sound; whereas, in the question, degree is asked of the same thing, i.e. sound.
Option (c) is wrong, because it just doesnt refer to the same relationship: painful refers to something that gives physical pain or distress, and prickly refers to something that is very irritable, which can be used to define someones nature; The degree relationship in the same sense, is not evident. Option (b) doesnt display the same relationship.