Explanation:

- World Environment Day is celebrated annually on June 5th to encourage awareness and action for environmental protection.

- The United Nations usually highlights a specific theme and designates a host country for the occasion each year.

- Option 1: China - Not the host for 2018. China later hosted in 2019.

- Option 2: USA - Incorrect. The USA did not host the event in 2018.

- Option 3: Britain - Britain was not the host country in 2018.

- Option 4: India - Correct. India was the host for World Environment Day in 2018. The theme was "Beat Plastic Pollution."

- Option 5: Switzerland - Switzerland was not the host for 2018.