Explanation

The way Vargas replies to Romley is condensed in the first two words: Not necessarily..

This means that he agree with the first statement but at the same time introducing new elements.

D wins
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More over He does provide counter example - which is there in the option choice D.

Vargas responds to Romley's argument by doing which of the following?

A) Arguing that Romley's argument applies only to a special case - opposite. As Vargas said "Not Necessarily". Meaning that it is not an exception
B) Contending that one of the dates introduced as evidence to support Romley's conclusion is wrong - Extreme. On the contrary, Vargas agrees somehow.
C) Contending that Romley's conclusion is inconsistent with generally accepted views of the history of medicine - Same as above
D) Proposing a counterexample to a generalization implicit in the formulation of Romley's argument - Vargas proposed a counter evidence to R's argument
E) Proposing a minor qualification of Romley's conclusion that would serve to shield it from a potential objection - Evidence is produced rather than qualification

Hence, D.