"what are the bacteria living in the ocean that do the nitrogen fixation, and were the first photosynthesizers on earth providing the early oxygen found in the atmosphere around 2 billion years ago

Cyanobacteria

Explanation:

Cyanobacteria were the first photosynthetic autotrophs that used water molecules as the source of electrons. Cyanobacteria were the first autotrophs that performed oxygenic photosynthesis and carried out the splitting of water.  

The process of splitting water released oxygen as a gas (O2) which was released in the atmosphere of the earth as a by-product. This led to the accumulation of oxygen in the atmosphere. Some cyanobacteria can fix nitrogen and have specialized cells called heterocyst which serve as the site for nitrogen fixation.  

the answer is c) an oak tree