The correct statement is:
A. 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 Richard Nixon's presidency, U.S. troop levels in Vietnam were reduced significantly as part of his policy of "Vietnamization," which aimed to decrease American involvement in the war. In contrast, the number of troops was higher at the beginning of his presidency.
Let's analyze the other options:
B. During Eisenhower's and Kennedy's administration U.S military involvement was the highest. - This is incorrect; the peak of U.S. military involvement in terms of troop numbers occurred during Johnson's presidency.
C. During Johnson's presidency the number of U.S military forces decreased. - This is incorrect; troop numbers increased significantly during Johnson's administration.
D. The least amount of troops in Vietnam occurred during Johnson's presidency. - This is incorrect; the least amount of troops occurred later, after the troop drawdown began under Nixon.
Therefore, the most accurate option is A.