Explanation:

The correct answer is option 'C' - economy.

Economy is defined as the number of kilograms of steam required to evaporate one kilogram of water in an evaporator. It is a measure of the efficiency of the evaporation process.

To understand why economy is the correct answer, let's break down the other options:

- Capacity: Capacity refers to the maximum amount of water that can be evaporated by the evaporator in a given time period. It is not a measure of efficiency but rather a measure of the size or capability of the evaporator.

- Rate of evaporation: The rate of evaporation refers to the amount of water that is evaporated per unit of time. It does not take into account the amount of steam required for the evaporation process.

- Rate of vaporization: The rate of vaporization also refers to the amount of water that is vaporized per unit of time. It is similar to the rate of evaporation and does not consider the steam input.

Therefore, the correct answer is economy which specifically measures the efficiency of the evaporation process by considering the amount of steam required to evaporate one kilogram of water. A higher economy value indicates a more efficient evaporation process, as less steam is required to achieve the same amount of evaporation.

In summary, while capacity, rate of evaporation, and rate of vaporization are related to the evaporation process, they do not specifically measure the efficiency of the process. The economy, on the other hand, provides a direct measure of efficiency by considering the amount of steam required per kilogram of water evaporated.