Final answer:
The correct MLA in-text citation for the referenced passage from 'Slaughterhouse-Five' is (Vonnegut 20).
Step-by-step explanation:
When referencing a specific passage from a text, proper citation is essential for academic writing integrity. In the case of the novel Slaughterhouse-Five by Kurt Vonnegut, when mentioning observations by the main character, Billy Pilgrim, about the abnormal movement of time in relation to a clock's hands, the correct in-text citation should include the author's last name and the page number without a comma. This practice aligns with the MLA (Modern Language Association) guidelines for in-text citations, a common style used in literature and humanities courses. The correct citation would be (Vonnegut 20).