The sun is the plant's most important nutrient. Plants convert sunlight into sugars in order to grow. Water is needed in two ways, it serves as both a solvent for mineral salts that are carried inside plant cells, and it is an essential component of photosynthesis. The questioner might have asked "name one" so they don't have enough information to answer with any greater certainty - but the answer remains the same regardless of how many nutrients they ask about.
Minerals are also required by plants in order to function properly including calcium, potassium, iron, magnesium just to name a few minerals which are found in healthy nutritious produce!
