Explanation:

All statements are correct.

• Geoengineering schemes are projects designed to tackle the effects of climate change directly, usually by removing CO2 from the air or limiting the amount of sunlight reaching the planet’s surface.

• The first category of scheme – those designed to remove CO2 from the air – include machines (sometimes called “artificial trees”) that pull the gas from the atmosphere using plastic polymers. Other proposals seek to increase the amount of CO2 absorbed by the oceans – for example by adding large quantities of lime to the water.

• In the second category – schemes designed to reduce the amount of sunlight reaching Earth – proposals include firing sulphate aerosols into the stratosphere to reflect sunlight back to space; using unmanned ships to increase above-ocean cloud cover by spraying sea water into the air; painting the world’s roofs white to increase reflectivity; and even floating thousands of tiny mirrors in space between Earth and the sun.

Hence option 4th is correct.