By 1969, the number of US troops in Vietnam was approximately 500,000.
To solve such a question, one would typically examine historical records of US military presence in Vietnam. Data from that period indicates that the United States had significantly ramped up its military involvement in Vietnam during the 1960s. By the peak of US involvement, which occurred in 1969, the troop levels reached around 500,000. Therefore, out of the given options, the correct answer is 500,000.
