The correct answer is option 2: Carbon-dioxide. Photosynthesis is the process by which plants convert light energy into chemical energy, and carbon dioxide is a crucial component for this process. In photosynthesis, plants use carbon dioxide from the air along with water from the roots to produce glucose (a type of sugar) and oxygen. The process occurs in the chloroplasts of plant cells, where sunlight is absorbed by the pigment chlorophyll. Through a series of chemical reactions, the plants use the carbon dioxide to build glucose molecules, which can be stored as energy or used for growth and reproduction. Oxygen is also produced as a byproduct of photosynthesis and is released into the atmosphere. While oxygen is essential for most living organisms, it is not directly required for plants to carry out photosynthesis.