Which of these are approaches to solar radiation management?

The correct approach to solar radiation management is reflected in the option involving high altitude reflective balloons and sulfur-seeded clouds. Solar radiation management (SRM) refers to geoengineering techniques aimed at reflecting a small proportion of the sun's light and heat back into space to counteract climate change and global warming.

High altitude reflective balloons can be deployed in the stratosphere to create a reflective barrier that reduces the amount of solar radiation reaching the Earth's surface. Similarly, injecting sulfur particles into the stratosphere can mimic the natural cooling effects observed after large volcanic eruptions, which release aerosols into the upper atmosphere. These aerosols scatter sunlight and contribute to a temporary cooling effect on the planet.

Both of these methods are direct interventions in the Earth’s radiative balance and are specifically categorized under solar radiation management strategies, which aim to counteract the effects of greenhouse gas-induced warming.

In contrast, the other options do not focus on manipulating solar radiation. Carbon offsets and emission trading are market-based mechanisms aimed at reducing greenhouse gas emissions. Subsurface irrigation methods are agricultural practices designed to conserve water and improve agricultural efficiency. Urban heat mitigation strategies often include urban greening and building design improvements that enhance energy efficiency and reduce heat buildup but do not actively manage solar radiation