Cyanobacteria a. are a form of algae. b. are prokaryotes. c. use hydrogen sulfide as an electron source. d. are eukaryotes.

Option B, are prokaryotes

Explanation:

Cyanobacterias are prokaryotic cells which possess cell organelles without membrane. Their structure is very much similar to the prokaryotic cells. They consists of chlorophyll and hence are able to photosynthesize. They use either water or  hydrogen sulfide as an electron source for ETS process of photosynthesis. The terminology “blue green algae” is used for phylum of bacteria known as Cyanobacteria , however they will be considered prokaryotic  primarily.  

Hence, option B is correct

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