The correct answer is C. June 5th 1972.
The Stockholm Conference on Human Environment was the first international conference on the environment. It was held in Stockholm, Sweden, from June 5 to 16, 1972. The conference was attended by representatives from 113 countries. The main outcome of the conference was the Declaration of the United Nations Conference on the Human Environment, which set out 26 principles for the protection of the environment. The conference also led to the establishment of the United Nations Environment Programme (UNEP).
Option A is incorrect because the Stockholm Conference on Human Environment was held in 1972, not 1974.
Option B is incorrect because the Stockholm Conference on Human Environment was held in 1972, not 1976.
Option D is incorrect because the Stockholm Conference on Human Environment was held in 1972, not 1990.