Assertion: All the Muslims migrated from India to Pakistan at the time of partition.
Reason: The partition of India had taken place on the basis of religion.

The Assertion is incorrect but the Reason is correct.

The partition of India refers to the division of British India into two separate nations: India and Pakistan. It occurred in 1947, following the end of British colonial rule in the Indian subcontinent. The partition was primarily based on religious lines, with the creation of a separate Muslim-majority state, Pakistan, to address the demands of the Muslim League for a separate homeland
Even after the large-scale migration of Muslims to the newly created Pakistan, the Muslim population in India accounted for 12 percent of the total population in 1951.