Final answer:
Theta proteobacteria is not a class within the phylum Proteobacteria. The recognized classes are Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, and Epsilonproteobacteria.
Step-by-step explanation:
The correct answer to the question 'which is NOT a member of the phylum Proteobacteria?' is option B) theta proteobacteria. The phylum Proteobacteria is subdivided into five classes: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, and Epsilonproteobacteria. These classes are well defined, and 'theta proteobacteria' is not recognized as one of them. In contrast, Epsilonproteobacteria is a valid class within the phylum Proteobacteria, which includes important human pathogens like Campylobacter and Helicobacter.