The correct answer is option 1, the Brundtland Report. The Brundtland Report, also known as Our Common Future, was published in 1987 by the World Commission on Environment and Development. It defines sustainable development as development that meets the needs of the present generation without compromising the ability of future generations to meet their own needs.

Let`s take a look at the other options to understand why they are not the correct answer:

Option 2, the United Nations Human Development Report, focuses on measuring and analyzing human development, but it does not specifically define sustainable development.

Option 3, the Agenda-21 of the Rio Earth Summit, is a comprehensive plan of action to be taken globally, nationally, and locally to promote sustainable development. While it outlines strategies and actions for achieving sustainability, it does not provide a specific definition.

Option 4, the OECD Policy Brief (2006), may discuss sustainable development, but it does not explicitly define it.

Therefore, the correct answer is the Brundtland Report as it is the report that provides a clear and widely accepted definition of sustainable development.