Final answer:
Sustainable energy production is an example of a hidden production function as it involves various inputs, but the exact relationship between these inputs and the output is not easily observable. The correct option c.
Step-by-step explanation:
A hidden production function refers to a production function where the relationship between inputs and outputs is not easily observable. It is not explicitly stated or defined. Of the given options, the best example of a hidden production function would be C. Sustainable energy production.
While sustainable energy production involves various inputs such as workforce, capital, and technology, the exact relationship between these inputs and the output of sustainable energy is not easily observable or defined. The correct option c.