The correct answer is: D. Serena Williams
Serena Williams has won 23 Grand Slam singles titles, the most of any player in the Open Era. She has also won 14 Grand Slam doubles titles and 2 Olympic gold medals in singles.
Venus Williams has won 7 Grand Slam singles titles, 5 Grand Slam doubles titles, and 2 Olympic gold medals in doubles.
Steffi Graf has won 22 Grand Slam singles titles, 10 Grand Slam doubles titles, and 1 Olympic gold medal in singles.
Margaret Court has won 24 Grand Slam singles titles, 11 Grand Slam doubles titles, and 3 Olympic gold medals in singles and doubles.
However, Court’s 24 Grand Slam singles titles were won before the Open Era, which began in 1968. In the Open Era, players from all over the world are eligible to compete in Grand Slam tournaments. This makes the Open Era titles more competitive and prestigious than the titles won before the Open Era.
Therefore, Serena Williams is the correct answer to the question of which woman tennis player has won the most Grand Slam titles.