Which combining form is used to denote work?

The combining form that denotes work is "erg/o." This term is derived from the Greek word "ergon," meaning "work." In medical terminology, "erg/o" is often used in terms related to energy expenditure and physical activity. For example, the term "ergometry" refers to the measurement of work or energy output, typically in a clinical setting, where understanding a patient's physical capabilities is important.

The other options represent different concepts. "Fet/o" pertains to the fetus in utero, "erythr/o" refers to red blood cells or the color red, and "esthesi/o" relates to sensation or feeling. Each of these has its own distinct meaning and usage within medical terminology and is unrelated to the concept of work. Thus, "erg/o" is the only combining form that accurately represents the idea of work in this context.