Why is the increase in atmospheric carbon dioxide dangerous?
a) the gas inhibits the escape of heat from earth’s atmosphere.
b) carbon dioxide inhibits rainfall causing long droughts.
c) the gas prevents plants from photosynthesizing.
d) carbon dioxide is highly toxic if inhaled.

Correct A) The gas inhibits the escape of heat from Earth’s atmosphere.  

The earth temperature depends upon the balance between the incoming radiation from the sunlight and the energy that is reflected back to space. Carbon dioxide absorbs the heat sent to earth by the sun, it acts as an insulator of the earth and helps to keep the atmosphere warm and make suitable for life process.

But, when the concentration of carbon dioxide in the atmosphere is increased, it means more and more increase in the temperature of the planet and cause global warming and make the life difficult on the planet. As a result of that whole life on the earth would be in danger.