Final answer:
The answer to the student's question is c. Solar energy, which is renewable and harnessed using photovoltaic cells but is not always available due to its intermittent nature.
Step-by-step explanation:
The unlimited energy supply that is not always available and is collected via photovoltaic cells is c. Solar energy. Solar energy is considered a renewable energy source because it comes from the sun, which provides an essentially limitless supply of energy that can be harnessed without the risk of depletion. Photovoltaic cells, also known as solar panels, are used to convert sunlight directly into electricity. Despite its unlimited supply, solar energy is intermittent; it is dependent on the time of day, weather conditions, and geographic location, which can limit its availability.