Final answer:
Efficiency refers to the effectiveness of energy input, while efficacy means doing a task correctly. In this question, the correct answer is effectiveness.
Step-by-step explanation:
Efficacy means doing a task correctly. Efficiency, on the other hand, refers to a measure of the effectiveness of the input of energy to do work. It is about how efficiently resources are used to produce desired goods and services.
In the context of the given question, effectiveness is the correct term. It signifies the successful accomplishment of a task or goal. For example, if a factory produces a large number of products, but many of them are defective, the efficiency may be high, but the effectiveness is low.
Therefore, the correct answer is c. effectiveness.