Which of the following is NOT required for photosynthesis to occur in plants?

• A. Oxygen gas ($O_2$)
• B. Light energy
• C. Water ($H_{2}O$)
• D. Carbon dioxide (${\mathrm{CO}}_2$)

Answer: (A)

Verified step by step guidance

1. Understand the overall process of photosynthesis, which is the conversion of light energy into chemical energy by plants.
2. Recall the general photosynthesis equation: \$6\,CO_2 + 6\,H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6\,O_2$.
3. Identify the reactants (inputs) required for photosynthesis: carbon dioxide (\(CO_2\)), water (\(H_2O\)), and light energy.
4. Recognize that oxygen gas (\(O_2\)) is actually a product (output) of photosynthesis, not a required input.
5. Conclude that oxygen gas is NOT required for photosynthesis to occur, while light energy, water, and carbon dioxide are essential.