Which of the following terms refers to a professional that studies grapes?

The term that refers to a professional who studies grapes is "viticulturist." Viticulture specifically focuses on the science of grape cultivation, which includes the growing of grapevines for winemaking and table grapes. Viticulturists work on various aspects of grape production, such as selecting grape varieties, managing vineyard health, and understanding the influence of climate and soil on grape quality. Their expertise is essential for the wine industry and influences the characteristics of the wine produced from the grapes they cultivate.

The other professions listed pertain to different areas of study: entomologists study insects, ornithologists focus on birds, and ichthyologists specialize in fish. Each of these fields has its relevance within biology and agriculture but does not pertain to the study or cultivation of grapes.