The correct answer is option 1, Stockholm. The Human Environment Conference, also known as the United Nations Conference on the Human Environment, was held in Stockholm in 1972. This conference marked the first major international meeting on environmental issues. It brought together representatives from numerous countries to address global environmental challenges and develop strategies for sustainable development. The conference resulted in the adoption of the Stockholm Declaration, which recognized the importance of environmental protection and called for collective action to address environmental problems. Stockholm was chosen as the venue for this conference due to its reputation as a city committed to environmental conservation and sustainability.