The substance on which the enzyme acts is called the
enzyme
substrate
catalyst
buffer

The best answer is substrate.

The molecule to which an enzyme binds is called a substrate. Substrates get specifically selected by enzymes because enzymes will only bond with the correct substrate that they have been genetically designed to bond with. 

The substrate binds to a small area on the enzyme referred to as the  active site and they form an enzyme - substrate complex.

Once the reaction between substrate and enzyme is complete, the substrate will have become a product.

b c a d mark me

explanation:

b,bacteria convert nitrates

into nitrogen gas.

d,plants and animals take in

and use nitrogen compounds.

e,bacteria break down waste    

products and return nitrogen  

to the soil.

a,bacteria convert atmospheric

nitrogen to ammonia.

c,bacteria convert ammonia

to nitrites and nitrates.