To determine which of these fitness events happened last, let's look at each option and their approximate dates:
A. PE becomes part of the American school curriculum - Physical Education began to be included in school curriculums in the late 19th to early 20th century, but a more formal recognition occurred around the 1960s.
B. Title IX forbids gender discrimination in sports - Title IX was enacted in 1972.
C. The YMCA opens a gym in America - The first YMCA in the U.S. was established in Boston in 1851, but it was not initially a gym.
D. The adjustable plate-loaded barbell is invented - While various forms of barbells have existed for a long time, adjustable plate-loaded barbells became popular in the 20th century, particularly around the 1950s and 1960s.
Based on this information, the event that occurred last is B. Title IX forbids gender discrimination in sports (1972).