To tell readers what they are looking at and why is the image is relevant
The answer to your question would be that the sentence that correctly uses an MLA in-text citation is the following one: Research done by fish biologist Sarah Myers suggests that 25 percent of the carp are carriers of the vius (25). That is, the correct option would be A.
In MLA, you refer to other texts by using parenthetical citation. In this way, relevant source information is between parentheses whenever a sentence uses a quotation or paraphrase. Usually, this is done by putting all the relevant information in parentheses at the end of the sentence. The information to be included should be the author's last name and the page number(s) from which the quotation or paraphrase is taken must appear in the text. Therefore, the author's name must appear in the sentence itself or in parentheses following the quotation or paraphrase, but the page(s) number should appear in the parentheses, which is the case here.
