Which of these men was a very aggressive and powerful leader of the teamsters union who was sentenced to serve a prison term and later kidnapped and has never been found?

The answer is Jimmy Hoffa. He was an American forerunner of Labor Union who served as the President of the IBT or International Brotherhood of Teamsters union from 1958 up until 1971. He disappeared in late July 1975, at age 62 and was declared legally dead in 1982. He turn out to be involved with organized crime from the initial years of his Teamsters work, and this linking sustained until his disappearance in 1975. He was sentenced of jury tampering, attempted bribery, and fraud in 1964, in two unconnected trials. He was held in 1967 and sentenced to 13 years. In middle of 1971, he stepped down as president of the union as part of a pardon agreement with President Richard Nixon; he was free later that year, although banded from union activities until 1980. Hoffa, eager to regain support and to return to IBT leadership, ineffectively attempted to overturn this order.