The correct answer is - A. A religious philosophy from India gains followers across the United States.
The cultural diffusion is term that is used when something derived form one culture is spread out and accepted in another culture.
In this case, we have a religious philosophy that has its origins in India, so it is part of India's culture and tradition. Despite being an Indian cultural trait, this religious philosophy found its way to the United States, and not just that, but it also was accepted by some people. So the people in the United States, despite being from another culture and religious background, found the Indian one more attractive, so they accepted it and started following it, thus making a perfect example for cultural diffusion.
