Carbon dioxide is a gas having chemical formula 
, a gas which exhaled by humans and inhaled by plants. It is also emitted from the sources such as cement production and combustion of fossil fuels. It is also known as greenhouse gas associated with acidification of ocean.
Carbon dioxide with other gases acts like a blanket of Earth which absorbs infrared radiations emitted from the sun and thus, preventing the escaping of these rays into the outer space which results in the gradual heating of Earth's surface and atmosphere i.e. increase in temperature of Earth. This process is known as Global Warming.
Hence, carbon dioxide is responsible for the increase in warming of the atmosphere i.e. option (D).
"Carbon" is an element. It is found in the fourth group of the periodic table, and it is a stable element. This means that it can not be decomposed via heating, because if an element were to break down, it would release its subatomic particles. The explanation was probably one used to describe the thermal decomposition of a compound into smaller compounds.
