Final answer:
Professor Umbridge used the textbook titled "Defensive Magical Theory" by Wilbert Slinkhard for Defense Against the Dark Arts class.
Step-by-step explanation:
The title and author of the textbook used by Professor Umbridge for Defense Against the Dark Arts (DADA) is C) "Defensive Magical Theory" by Wilbert Slinkhard. Umbridge, who was appointed as the DADA professor in Harry Potter's fifth year at Hogwarts, favored a theoretical, non-practical approach to magic, which was reflected in the content of this book. This textbook was heavily criticized by the students, including Harry, because it did not provide practical defense techniques against dark forces.