The correct answer is (b), 1972.
The United Nations Conference on the Human Environment, also known as the Stockholm Conference, was the first international conference on the human environment. It was held in Stockholm, Sweden, from June 5 to 16, 1972. The conference was attended by representatives from 113 countries and 17 international organizations.
The conference adopted a declaration and an action plan, which called for international cooperation to protect the environment. The declaration also established the United Nations Environment Programme (UNEP).
The Stockholm Conference was a landmark event in the history of environmental protection. It was the first time that the international community had come together to address the issue of environmental degradation. The conference helped to raise awareness of the importance of protecting the environment and to promote international cooperation on environmental issues.
The options (a), (c), and (d) are incorrect because they are not the years in which the Stockholm Conference was held.