Which of the following refers to a tangible item of evidence?

The term "exhibit" specifically refers to a tangible item of evidence presented during a legal proceeding or trial. This could include documents, photographs, physical objects, or any item that can be presented to support a case or argument being made. Exhibits play a critical role in trials as they help to establish facts and provide credibility to the testimonies given by witnesses.

In contrast, an "expert" pertains to an individual who possesses specialized knowledge, skill, or experience in a particular area relevant to the case, but does not refer to physical evidence itself. A "federal question" pertains to issues that arise under federal law, indicating the jurisdiction of federal courts, without relating to evidence or physical items. "Gravamen" refers to the essence or foundation of a legal complaint, emphasizing the core issue rather than tangible evidence.

Thus, the choice of "exhibit" directly aligns with the concept of a physical piece of evidence used in legal contexts, marking it as the most appropriate option.