Answer : 2. "5 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.

4. The issue of celebrating World Environment Day was raised for the first time in the United Nations General Assembly regarding human environment. After this, on June 5, 1973, many countries celebrated the first World Environment Day. You should read the Environment Day facts listed below.