The scenario described involves the partition of India in 1947, which led to the displacement of approximately 17 million individuals. This was primarily a result of religious and ethnic tensions, with large numbers of Muslims moving to Pakistan and Hindus and Sikhs migrating to India.
Given this context, the most appropriate reason for the migration is:
forced political migration.
This type of migration occurs when individuals are compelled to leave their homes due to political reasons, such as conflict, war, or the division of territories, which is precisely the case during the partition.