The amount of carbon on the Earth, including the atmosphere,

a. is essentially constant

b. is decreasing due to consumption of carbon based fuels, such s coal and petroleum

c. is increasing due to plant and animal growth on the planet

d. fluctuates dramatically with the seasons

Option (A)

Explanation:

Carbon is one of the essential elements, that plays an important role on the earth's surface. It forms the fundamental unit for the existence of life. Without carbon, life is not possible. This carbon moves from one sphere to another, by different processes and are constantly balanced in the environment.

The carbon is released into the atmosphere, from the animals and humans in the form of CO₂, from the volcanic eruptions, and burning up of fossil fuels. Again plants absorb these CO₂, where the food produced by the plants is again consumed by the consumers. As these consumers die, some of the carbon stored in them are consumed by the decomposers, and the remaining are mixed with the environment.

This is how the carbon is transferred from one place to another, maintaining a cycle, and the amount of carbon including the atmosphere is essentially constant.

Thus, the correct answer is option (A).