What is the region called where roots separate when teeth have more than one root?

The correct answer is "Furcation." This term specifically refers to the area where the roots of a multirooted tooth diverge or separate. In dentistry, understanding furcation is crucial because it can be a site for periodontal disease or other dental issues due to difficulty in cleaning the area properly.

For teeth with two or more roots, the furcation area allows for the support of the tooth by anchoring it into the jawbone. This anatomical feature is significant in dental hygiene practices, as well as in assessing periodontal health. Other terms included in the question refer to different structures or concepts: "cusp" refers to the pointed part of a tooth where chewing occurs; "medullary" generally pertains to the innermost part of a bone; and "apical" relates to the tip of the root of a tooth. Recognizing these distinctions is vital for anyone studying dental anatomy or engaged in the field of dentistry.