Definitions: The ratio of the heat extracted in a refrigerator to the work input to the refrigerant is called the __________.

Introduction / Context:
Performance metrics distinguish refrigerators, freezers, and heat pumps. The most common is the coefficient of performance (C.O.P.), which appropriately measures benefit per unit input for devices that move heat rather than convert it to work.

Given Data / Assumptions:

• Refrigerator objective: remove heat Q_L from a low-temperature space.
• Work input to the cycle is W.
• Steady-state operation, negligible kinetic/potential energy changes.

Concept / Approach:
Define C.O.P._ref = Q_L / W. This differs from a heat pump's C.O.P._hp = Q_H / W, which counts heat delivered to the warm space. The relative C.O.P. normalizes actual C.O.P. by the Carnot limit, but the basic definition asked here is simply Q_L divided by W.

Step-by-Step Solution:

Verification / Alternative check:
For a Carnot refrigerator, C.O.P._Carnot = T_L / (T_H − T_L), which when multiplied by W yields Q_L, reinforcing the definition.

Why Other Options Are Wrong:

• Heat pump C.O.P. uses Q_H, not Q_L.
• Relative C.O.P. is a ratio to the Carnot value, not the base definition.
• “Refrigerating efficiency” is an informal term and ambiguous here.

Common Pitfalls:
Swapping numerators between refrigerator and heat pump; using Celsius instead of kelvin in Carnot expressions.

Final Answer:
coefficient of performance of refrigeration (C.O.P.)