Correct Option
The correct option isSunlight
Explanation
Photosynthesis is the physicochemical process by which green plants utilize light energy to synthesize organic compounds. The essential inputs for this process are sunlight, chlorophyll, water, and carbon dioxide. In plant physiology, a "nutrient" is defined as a chemical element or substance required for plant growth, metabolism, and external supply.
Detailed Analysis
- Air: Plants obtain Carbon Dioxide ($CO_2$) from the air. Carbon and Oxygen are essential non-mineral nutrients derived from air (and water). Therefore, air acts as a source of nutrients.
- Water: Water ($H_2O$) is absorbed by roots and provides Hydrogen, which is an essential non-mineral nutrient. It also acts as an electron donor in the light reaction of photosynthesis.
- Soil: Soil is the primary reservoir for mineral nutrients such as Nitrogen, Phosphorus, Potassium, Magnesium, and others, which are absorbed by plant roots.
- Sunlight: Sunlight provides the energy (photons) necessary to drive the chemical reactions of photosynthesis. While it is absolutely essential for the process, it is a form of energy, not a chemical substance or element. Therefore, it is not classified as a nutrient.
Key Takeaway: Nutrients are material substances (chemical elements) required for plant metabolism. Sunlight is the external energy source that powers the synthesis of food but does not provide material nourishment itself.