Question 1
Carbon exists in the atmosphere primarily in the form of:

(a) Carbon monoxide only
(b) Carbon monoxide and carbon dioxide
(c) Carbon dioxide only
(d) Methane

(b) Carbon monoxide and carbon dioxide.
Carbon in the atmosphere primarily exists in the form of its oxides - Carbon monoxide ($\text{CO}$) and Carbon dioxide (${\text{CO}}_2$).
Carbon monoxide is a colourless, odourless, tasteless gas which is released due to incomplete combustion of carbon.
Carbon dioxide is a colourless, acidic gas which plays a big role in controlling earth's surface temperature through the greenhouse effect. Both these gases are present in low quantities.