Many spirochetes are difficult to cultivate, so their classification is based on their a. morphology and ability to cause disease. b. number of flagella. c. pattern of pili. d. number of chromosomes.

A. Morphology and Ability to Cause Disease

Explanation:

Spirochetes have a lot of requirements that make them a difficult organism to cultivate in traditional mediums, that's why previously they were classified base on their morphology (appearance) and if these organisms were able to cause disease in humans. The current Classification uses DNA studies like PCR to categorize the bacteria.

bacterium is not correct