What is the key organism that ensures the
nitrogen cycle can run properly?

the nitrogen fixing bacteria is an important organims in the nitrogen cycle which helps .in fixing the nitrogen into the soil  

Explanation:

carbohydrates: (starches and sugars) composed of carbon, hydrogen, and oxygen.

lipids: (fats, oils, and waxes) are made of carbon and hydrogen atoms.

nucleic acids: contain hydrogen, oxygen, nitrogen, carbon, and phosphorus. they are polymers of nucleotides. a nucleotide has three parts, a 5-carbon sugar, a phosphate     (-po⬇️4) group, and a nitrogenous base.

proteins: made up of nitrogen, carbon, hydrogen, and oxygen. they are polymers of amino acids.

if you listed your answers right then i believe your answer is b.