Sustainable development is a crucial concept in contemporary environmental and economic discourse. The concept was introduced by the Brundtland Report, formally known as "Our Common Future," which was published in 1987 by the World Commission on Environment and Development (WCED).

Let's analyze the options to understand why the Brundtland Report is the correct answer:

1. Rio Declaration: This was part of the United Nations Conference on Environment and Development (UNCED) held in Rio de Janeiro in 1992. It focused on international policies for sustainable development but was not the original source of the term 'sustainable development'.
2. Johannesburg Declaration: This came out of the World Summit on Sustainable Development in 2002, which revisited and expanded on the concepts from the Rio Conference, but again, it was not the origin of the concept.
3. Agenda 21: Also a product of the 1992 Earth Summit in Rio, it is a comprehensive plan of action to build a global partnership for sustainable development but not where the term was first introduced.
4. Brundtland Report: This report introduced the well-known definition of sustainable development — "development that meets the needs of the present without compromising the ability of future generations to meet their own needs." It highlighted the need for modern development practices that conserve environmental resources.

Therefore, the correct answer is the Brundtland Report, which laid the foundational framework for sustainable development as recognized and implemented globally.

Conclusion: The concept of 'sustainable development' was first introduced by the Brundtland Report, making option D the correct choice.