To be clear, the original version is wrong.
The use of a pair of commas would make it nonessential information and work with the indefinite article.
. . . by an American scientist, Jonas Salk, in 1955 . . .
. . . by an American scientist in 1955 . . .
The name adds information to the sentence, but it is not a requirement to the sentence structure.
To make it essential information, there should be no commas but there should also be no indefinite article (either the definite article or no article at all):
. . . by the American scientist Jonas Salk in 1955 . . .
. . . by American scientist Jonas Salk in 1955 . . .
Essential information, in this sense, is a grammatical phrase. If you think that the name of the scientist is essential to the meaning you want to convey, then you should not be putting the name inside a pair of parenthetical commas. (However, in this case, that wasn't one of the possible answers.)