Answer : 3. "5th June "
Explanation :

1. World Environment Day is celebrated every year on 5 June.

2. Its objective is to make mankind aware of the environment.

3. First held in 1973, it has been a platform to raise awareness on environmental issues such as marine pollution, overpopulation, global warming, sustainable development and wildlife crime. World Environment Day is a global platform for public outreach, with participation from over 143 countries annually.