Scientists believe were the first types of cells able to make their own food.
a. cyanobacteria
b. chemoautotrophs
c. photoautotrophs

a. cyanobacteria

Explanation:

Cynobacteria is considered to be the first organism on earth which was capable of producing it's own food by the process of photosynthesis. The cynobacterial strain used to exists on earth around 3.5 billion years ago. These were the first unicellular prokaryotic organisms. They had a large number of colonies. These were responsible for the liberation of large amount of oxygen in the oxygen deficient primitive earth atmosphere.    

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products