The increase of Buddhist numbers in North America during the 1970s was primarily due to Vietnamese immigration after the Vietnam War in 1975, which introduced many Buddhist traditions. While other factors like the arrival of influential monks also played a role, the impact of Vietnamese refugees was more substantial. Therefore, the correct answer is option A: Vietnamese immigration into the United States after the end of the Vietnam War.
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The increase of Buddhist numbers in North America in the 1970s was primarily due to Vietnamese immigration following the Vietnam War. This influx brought significant Buddhist traditions and practices to the region. While other events also contributed, such as the arrival of influential monks, the impact of Vietnamese refugees was the most substantial. ;