Final answer:
The U.S. Secretary of State who rejected Senator Robert Kennedy's Vietnam peace plan in March 1967 was Dean Rusk.
Step-by-step explanation:
The Secretary of State who rejected Robert Kennedy's Vietnam peace plan was Dean Rusk. In March 1967, Kennedy, as United States Senator from New York, proposed a peace plan aimed at resolving the ongoing and escalating conflict in Vietnam. However, his plan was not accepted by Dean Rusk, the then-serving U.S. Secretary of State under the administrations of Presidents John F. Kennedy and Lyndon B. Johnson.
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