The question asks to identify the event celebrated on December 1st.

December 1st Significance: World AIDS Day

December 1st is internationally recognized and observed every year as World AIDS Day. This day serves as an important occasion to unite people worldwide in raising awareness about HIV/AIDS, showing support for people living with HIV, and commemorating those who have died from the disease. While the option mentions "National AIDS day", the globally accepted term is World AIDS Day, and December 1st is the established date for this observance.

Analysis of Other Options

Let's look at why the other options are incorrect for December 1st:

• World Environment Day: This day is celebrated on June 5th annually. It focuses on environmental protection and raising awareness about global environmental issues.
• World Women Day: Commonly known as International Women's Day, this is celebrated on March 8th. It marks the social, economic, cultural, and political achievements of women.
• National Child Day: The date for National Child Day varies significantly depending on the country. For instance, it is celebrated on November 14th in India and November 20th in many other countries as Universal Children's Day. It is not associated with December 1st.

Therefore, the event commemorated on December 1st is related to AIDS awareness.