The correct answer for the short name "Rio + 20" is option 2, which is the United Nations Conference on Sustainable Development. The event is commonly referred to as Rio + 20 because it took place in Rio de Janeiro, Brazil, in the year 2012, marking the 20th anniversary of the original Earth Summit held in the same city in 1992.

Option 1, Millennium Development Goals, is not the correct answer. The Millennium Development Goals were a set of eight global development targets established by the United Nations in 2000, which aimed to address poverty, education, health, gender equality, and other key issues. While the Rio + 20 conference discussed sustainable development, it was not directly related to the Millennium Development Goals.

Option 3, Earth Summit, is partly correct. The original Earth Summit held in 1992 in Rio de Janeiro was the event where the concept of sustainable development was formally introduced and various environmental agreements were signed. However, Rio + 20 specifically refers to the United Nations Conference on Sustainable Development that took place in Rio de Janeiro in 2012.

Option 4, Post-2015 Development Agenda, is incorrect. The Post-2015 Development Agenda refers to the global framework for development goals that succeeded