Final answer:
The correct in-text citation for a sentence about Lincoln's Gettysburg Address should include the author's last name and page number in parentheses, or the author's last name if no page number is available.
Step-by-step explanation:
The proper way to make an in-text citation for the sentence about Lincoln's Gettysburg Address, when referencing from the biography, and if page numbers are available, would be to include the author's last name and the page number where the information can be found in parentheses right after the quote. For example, "Lincoln's speech at Gettysburg is perhaps the greatest example of speech writing" (Smith 23). If no page number is available, you would include the author's last name only. In a situation where the document is online and no page numbers are provided, such as the case of the Gettysburg Address itself, and if paragraphs are too numerous to count, use just the author's last name (Lincoln).