This question checks a specific UN observance date. April 20, April 21 and April 23 are all close to the real date but none of them is correct, so they can be dismissed as a group.
The United Nations fixed April 22 as International Mother Earth Day in 2009, building on the Earth Day tradition that started in 1970, to draw attention to environmental protection and sustainable living worldwide.
So the correct answer is April 22.
Tracing where this observance came from makes the correct date easier to pin down than simply memorising a number. The tradition begins with the original Earth Day, first organised in the United States in 1970 as a grassroots environmental movement.
The line from the 1970 grassroots Earth Day to its 2009 formal adoption by the UN lands on one specific date out of the four offered.
Therefore, the correct answer is April 22.