This experiment was performed to check the role of carbon dioxide released during respiration after some time the water level in the bent tube rises since plants use sunlight and carbon dioxide present in the air to create food for themselves.
Option A is incorrect. One glucose molecule is converted to two pyruvate molecules during glycolysis, creating two net ATP and two NADH. The two pyruvate molecules are first converted to two acetaldehyde molecules during NADH regeneration, releasing two carbon dioxide molecules during the process.
Option B is incorrect. Glucose and oxygen react in cellular respiration to form ATP. This releases water and carbon dioxide as by-products. Glycolysis (an anaerobic process), the Krebs cycle, and oxidative phosphorylation are the three stages of aerobic cellular respiration.
Option C is correct. Moll's experiment with the half-leaf shows that unless there is carbon dioxide available, plants cannot photosynthesize. Plants use sunlight and carbon dioxide present in the air to produce food for themselves, becoming adversely affected in the absence of either photosynthesis. He placed a portion of a destarched plant's leaf into a conical flask containing potassium hydroxide. Potassium hydroxide solution absorbs the gaseous carbon dioxide from the air in the glass container.
Option D is incorrect. In photosynthesis, chlorophyll is important so plants can absorb energy from light. In the blue portion of the electromagnetic spectrum as well as in the red portion chlorophylls absorb light most strongly.
So, the correct answer is Option C.
Note: Moll's half-leaf experiment concluded that requirements for photosynthesis include CO$_2$, light, chlorophyll, and water. The part of the leaf kept inside the bottle does not touch the solution of potassium hydroxide. Applying grease or vaseline should keep the device airtight.