21 September is observed as –

World day for War Orphans
United Nations Day for South-South Cooperation
Labour Day
World Alzheimer's Day

The correct answer is: World Alzheimer’s Day.

World Alzheimer’s Day is observed on September 21 every year to raise awareness of Alzheimer’s disease and other dementias. The day was first observed in 1994, and it is now celebrated in over 100 countries around the world.

Alzheimer’s disease is a progressive, neurodegenerative disorder that affects the brain. It is the most common form of dementia, accounting for 60-80% of all cases. Dementia is a general term for a decline in mental ability that is severe enough to interfere with daily life.

There is no cure for Alzheimer’s disease, but there are treatments that can help to manage the symptoms. These treatments include medications, lifestyle changes, and support services.

World Alzheimer’s Day is an opportunity to learn more about Alzheimer’s disease and other dementias, and to show support for those affected by these conditions.

Here is a brief description of each option:

  • World day for War Orphans is observed on June 6 every year. The day was established by the United Nations General Assembly in 2002 to raise awareness of the plight of war orphans and to promote their rights and well-being.
  • United Nations Day for South-South Cooperation is observed on December 19 every year. The day was established by the United Nations General Assembly in 1986 to promote cooperation among developing countries.
  • Labour Day is a public holiday in many countries around the world. The day is observed on different dates in different countries, but it is usually celebrated on the first Monday of May. Labour Day is a day to commemorate the achievements of the labour movement and to celebrate the rights of workers.
  • World Alzheimer’s Day is observed on September 21 every year. The day was first observed in 1994, and it is now celebrated in over 100 countries around the world.