Final answer:
Cyanobacteria can use O2 as an electron source because of the presence of thylakoid membranes, which facilitate oxygenic photosynthesis.
Step-by-step explanation:
The question pertains to why cyanobacteria can use O2 as an electron source. Cyanobacteria are known for their abilities to conduct oxygenic photosynthesis, which is the process where water is oxidized, releasing oxygen and providing electrons. The correct answer to this question is that they can use O2 due to the presence of thylakoid membranes, similar to those found in plants, where the photosynthetic apparatus is located. Unlike the other options provided, a) their lack of chlorophyll b) the presence of phycobilins, and d) the possession of nitrogen-fixing enzymes, do not directly enable cyanobacteria to use O2 as an electron source for photosynthesis.