Heterocysts a. are found in nitrogen-fixing cyanobacteria. b. are used to protect nitrogenase. c. produce catalase. d. generate oxygen. e. are found in nitrogen-fixing cyanobacteria and are used to protect nitrogenase.

E. are found in nitrogen-fixing cyanobacteria AND are used to protect nitrogenase.

Explanation:

Heterocysts are specialized cellular compartments found in nitrogen fixing cyanobacteria. They provide the ideal anaerobic environment for nitrogen fixation since nitrogenase enzyme cant work in presence of oxygen. Hence, in a cyanobacteria both the oxygen requiring photosynthesis process and the  oxygen sensitive nitrogen fixation process takes place simultaneously.

Heterocysts have an additional cell wall made of glycolipid that provides a hydrophobic barrier to oxygen. They are involved in production of nitrogenase and other proteins that are required for nitrogen fixation. They also degrade photosystem II which is responsible for producing oxygen and produce proteins which remove any remaining oxygen.