What term describes a person with expert knowledge or training, particularly in fine arts?

The term that describes a person with expert knowledge or training, particularly in fine arts, is "connoisseur." A connoisseur has a deep appreciation and understanding of the nuances and subtleties within a specific field, such as art, wine, or cuisine. This expertise allows them to make informed judgments and evaluations regarding quality and value.

In contrast, the other terms do not align with the definition provided in the question. "Contentious" refers to something likely to cause an argument or controversy, which does not pertain to expertise or appreciation in a subject. "Compliant" denotes an inclination to yield or agree with rules or requests, which is unrelated to expertise in the arts. Lastly, "covenant" refers to a formal agreement or contract, rather than an individual with specialized knowledge or experience in a particular area. Thus, "connoisseur" stands out as the correct choice for someone knowledgeable about fine arts.