Explanation:

- The Bharat Series (BH) of vehicle registration was introduced to facilitate seamless transfer of vehicles across states in India.

- It particularly benefits people who move frequently across states, like government employees and military personnel.

- The Bharat Series allows vehicles to have a pan-India registration, thereby eliminating the need for re-registration upon moving to a new state.

Let's evaluate the options:

- Option 1: 15th March 2021: This date is earlier than the actual launch of the Bharat Series.

- Option 2: 15th October 2021: This is after the actual launch date.

- Option 3: 15th January 2021: This date is also before the rollout.

- Option 4: 15th September 2021: This is the correct date for the launch.

- Option 5: 15th February 2021: Again, too early for the launch date.