A renewable energy source.

**The reasoning** A renewable energy source is one that **naturally replenishes itself** and won't run out with human use. Solar energy comes from the sun, which shines every day and will continue doing so for billions of years. We can harness it through solar panels without depleting it. This fits the definition perfectly — it's sustainable and inexhaustible on human timescales. **Why the wrong options tempt you** Coal, oil, and gas are all **fossil fuels**. They took millions of years to form from ancient plant and animal remains. Once we burn them, they're gone — we can't replace them in our lifetime or even in thousands of years. Students often pick these because they're familiar energy sources used in Nigeria (like petrol or diesel), but familiarity doesn't mean renewable! **Quick takeaway** If nature can't replace it within your lifetime, it's **not renewable** — only sources like solar, wind, and hydro (water) keep coming back naturally.