To solve this problem, we need to understand the purpose of the Earth Summit held in Rio de Janeiro in 1992. Let's analyze the options:$\newline \bullet$Option 1: To reduce $CO_2$ emissions and global warming.$\newline$ - While reducing $CO_2$ emissions and addressing global warming are important environmental issues, they were not the primary focus of the 1992 Earth Summit.$\newline \bullet$Option 2: For conservation of biodiversity and sustainable utilization of its benefits.$\newline$ - The Earth Summit, also known as the United Nations Conference on Environment and Development (UNCED), focused on sustainable development and included the Convention on Biological Diversity, which aimed at the conservation of biodiversity and sustainable use of its components.$\newline$ - This matches the purpose of the Earth Summit.$\newline \bullet$Option 3: To assess threat posed to native species by invasive weed species.$\newline$ - Although invasive species are a concern for biodiversity, this specific issue was not the central theme of the Earth Summit.$\newline \bullet$Option 4: For immediate steps to discontinue use of CFCs that were damaging the ozone layer.$\newline$ - The issue of CFCs and the ozone layer was addressed by the Montreal Protocol, not the Earth Summit.$\newline$Therefore, the correct option is Option 2: For conservation of biodiversity and sustainable utilization of its benefits.