Explanation:

Here’s how it really breaks down:

- World Environment Day (WED) was first celebrated in 1973.

- The idea was established after the United Nations Conference on Human Environment held in Stockholm in 1972.

- 1972 is when the UN decided on the date and purpose, but the first actual celebration took place the following year.

- 1971 is too early—environmental awareness was just gaining global traction then.

- 1981 is way too late, by then WED was already pretty established.

- 1961 predates the environmental movement’s organized efforts at a global level.

So, the correct answer is option 1: 1973.