Final answer:
To introduce a short, direct quotation in the form of a complete sentence, a comma is used after the signal phrase, followed by the quotation enclosed in quotation marks.
Step-by-step explanation:
The punctuation mark used to introduce a short, direct quotation in the form of a complete sentence is a comma, usually following a signal phrase. When integrating quotations into your writing, it is important to maintain clarity and avoid plagiarism. If the direct quotation is a complete sentence, a comma should be placed after the signal phrase to introduce the quotation, and then quotation marks are used to enclose the quoted material. For example: Smith says, "90% of A and B students begin their essays at least two weeks before they are due." It's essential to ensure that periods and commas are placed inside the quotation marks when they are part of the quoted material.