The statement that correctly compares the involvement in the Vietnam War during different presidencies is:
D. The number of troops at the end of Nixon's presidency was less than the number of troops at the beginning of his presidency.
During Nixon's presidency, there was a process of "Vietnamization," which aimed to reduce U.S. troop levels in Vietnam while transferring responsibility for the war to the South Vietnamese forces. As a result, the number of U.S. military personnel in Vietnam decreased throughout his time in office.