The United Nations Conference on Environment and Development (UNCED), which adopted 'Agenda 21', was held at:

The United Nations Conference on Environment and Development (UNCED), popularly known as the "Earth Summit" or "Rio Summit", was held in Rio de Janeiro, Brazil, from 3 to 14 June 1992.

About the Earth Summit (1992):

It was held on the 20th anniversary of the first UN conference on the environment, held in Stockholm in 1972.

It was attended by representatives of 172 countries, including more than 100 heads of state or government, and thousands of non-governmental organisations. It was one of the largest gatherings of world leaders at that time.

Its main theme was "sustainable development": development that meets the needs of the present without compromising the ability of future generations to meet their own needs (a definition from the Brundtland Report, "Our Common Future", 1987).

Its main outcomes:

• Agenda 21: a detailed, non-binding action plan for sustainable development in the 21st century, at the global, national and local levels.
• Rio Declaration on Environment and Development: 27 principles, including the "precautionary principle", the "polluter pays principle" and "common but differentiated responsibilities".
• Forest Principles: a non-binding statement on the management of forests.

Two legally binding conventions were opened for signature:

• The UN convention on climate change (UNFCCC).
• The Convention on Biological Diversity (CBD).
• It also started negotiations that led to the UN Convention to Combat Desertification (UNCCD, 1994).
• The UNFCCC, CBD and UNCCD are together called the "Rio Conventions".
• The "Commission on Sustainable Development" was set up to monitor the progress of Agenda 21.

Follow-up summits:

• Rio+5 (1997): New York.
• World Summit on Sustainable Development (Rio+10, 2002): Johannesburg, South Africa.
• Rio+20 (UN Conference on Sustainable Development, 2012): Rio de Janeiro; its outcome document was titled "The Future We Want", and it started the process that led to the Sustainable Development Goals (2015).

Other important environmental conferences:

• Stockholm Conference (1972): led to the creation of UNEP and World Environment Day.
• Kyoto Protocol (1997): set binding emission targets for developed countries.
• Paris Agreement (2015): under the UNFCCC (COP21).

Why not the other options?

• ✗Stockholm hosted the first UN environment conference in 1972.
• ✗Kyoto gave its name to the 1997 Kyoto Protocol.
• ✗Johannesburg hosted the World Summit on Sustainable Development in 2002.