A is the most reasonable answer, Presidents Eisenhower and Carter were not president during the US involvement in Vietnam and Nixon's Vietnam plan stated for an increase in troops to the country.
The correct option is "President Nixon announced a Vietnamization plan for withdrawal of U.S. troops.
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President Nixon outlined a plan called Vietnamization, which consisted of a process to withdraw US troops from Vietnam, while fighting was returning to the Vietnamese in the south. The withdrawal of US troops began in July 1969.
