Question

which form of matter is most relevant to the carbon cycle?
a co₂
b h₂o
c po₄
d n₂

Explanation:

The carbon - cycle involves the movement of carbon through the Earth's atmosphere, biosphere, hydrosphere and geosphere. Carbon dioxide ($CO_2$) is a key component as it is exchanged between the atmosphere and living organisms during processes like photosynthesis and respiration. Water ($H_2O$) is mainly involved in the water - cycle, phosphate ($PO_4$) in the phosphorus - cycle and nitrogen gas ($N_2$) in the nitrogen - cycle.

Answer:

A. $CO_2$