The headquarter of the United Nations Environment Programme (UNEP) is located in Nairobi, Kenya. Here is a detailed explanation of this answer:

Introduction:
The United Nations Environment Programme (UNEP) is a specialized agency of the United Nations (UN) that focuses on environmental issues and promoting sustainable development. It was established in 1972 and is headquartered in Nairobi, Kenya.

Reasons for Nairobi, Kenya being the headquarters:
1. Geographical Location: Nairobi is strategically located in Africa, which allows UNEP to have a strong presence and focus on the continent's environmental challenges. Africa is home to diverse ecosystems, wildlife, and natural resources, making it a crucial region for environmental conservation and sustainable development efforts.

2. Environmental Significance: Kenya is known for its rich biodiversity, including iconic wildlife such as lions, elephants, and rhinos. The country has various national parks, game reserves, and protected areas that highlight the importance of environmental conservation. By having its headquarters in Nairobi, UNEP can directly engage with local environmental issues and collaborate with African countries.

3. Partnerships and Collaborations: Nairobi serves as a hub for many international organizations and institutions focused on conservation and sustainable development. Being located in the same city allows UNEP to foster partnerships, exchange knowledge, and collaborate with these organizations effectively. It also provides opportunities for joint initiatives and projects with local and regional stakeholders.

4. UN Presence: Nairobi is one of the major UN hubs in Africa, hosting several other UN agencies and programs. This concentration of UN entities facilitates coordination, synergies, and collective efforts towards achieving the UN's Sustainable Development Goals (SDGs). It also allows UNEP to work closely with other UN bodies and benefit from the shared resources and expertise available in the city.

Conclusion:
In conclusion, the headquarters of the United Nations Environment Programme (UNEP) is located in Nairobi, Kenya. This choice is based on Nairobi's strategic location, environmental significance, partnerships, collaborations, and the presence of other UN agencies in the city. By being situated in Nairobi, UNEP can effectively address Africa's environmental challenges and contribute to global efforts in promoting sustainable development.