The "Agenda-21" of Rio Summit, 1992 is related to:

1. A

   Sustainable development

   Option (a) claims that Agenda-21 of the Rio Summit, 1992 is related to sustainable development. This option is selected as the official key because it comprehensively encapsulates the core objective of the Rio Earth Summit, which was a non-binding action plan of the United Nations with regard to sustainable development, detailing a comprehensive blueprint of action to be taken globally, nationally, and locally by organizations of the UN system, governments, and major groups in every area in which human impacts on the environment occur.

2. B

   Polluter-Pays principle

   Option (b) claims that Agenda-21 relates to the Polluter-Pays principle. While the Rio Declaration did address economic polluter principles in Principle 16, Agenda-21 itself is a broader comprehensive action plan specifically focused on global sustainable development.

3. C

   Environmental education

   Option (c) claims that Agenda-21 pertains to environmental education. Although education for sustainable development is a minor component within the vast scope of Agenda-21, it is not the defining core subject of the entire document.

4. D

   Preservation of ozone layer

   Option (d) claims that Agenda-21 is about the preservation of the ozone layer. Ozone layer protection was primarily addressed by the Vienna Convention and the Montreal Protocol, not the Rio Summit's Agenda-21.

Summary. Official key is (a) Sustainable development. The Rio Summit, officially known as the United Nations Conference on Environment and Development held in 1992, adopted Agenda-21 as a global action plan. This extensive blueprint addresses economic, social, and environmental dimensions of development. Other options like polluter pays, environmental education, and ozone preservation relate to different environmental instruments or narrow aspects. Therefore, option (a) stands out as the comprehensive and correct answer.