Option b is the correct answer. The institution of child marriage started in the Kushan period. The Kushans were a Central Asian people who ruled large parts of India from the 1st to the 3rd centuries CE. They were influenced by the cultures of both India and Central Asia, and they brought with them many new ideas and practices, including the practice of child marriage. The practice of child marriage was not common in India before the Kushan period. However, it became increasingly common during the Kushan period and in the centuries that followed. There were a number of reasons for this, including the belief that child marriage would ensure the purity of the bride and groom, the desire to secure family alliances through marriage, and the economic benefits of having a son at a young age.