It is due to photo respiration (C2 cycle) i.e
at high O2 conc. 6 ribulose start forming some waste chemicals which decreases the rate of photosynthesis.

The correct answer is option 'C', the Warburg effect.

Explanation:
The Warburg effect refers to the decreased rate of photosynthesis at high concentrations of oxygen (O2). Let's understand this effect in detail:

1. Warburg Effect:
- The Warburg effect is a phenomenon observed in plants where the rate of photosynthesis decreases when the concentration of oxygen exceeds a certain level.
- It is named after the German physiologist and Nobel laureate, Otto Heinrich Warburg, who first described this effect in the 1920s.
- The Warburg effect is also known as "oxygen inhibition" or "photorespiration."

2. Photosynthesis:
- Photosynthesis is the process by which plants convert sunlight, water, and carbon dioxide (CO2) into glucose and oxygen.
- It occurs in specialized cell structures called chloroplasts, specifically in the thylakoid membranes.

3. Oxygen and Photosynthesis:
- Oxygen is one of the products of photosynthesis. It is released as a byproduct when plants generate glucose using sunlight, water, and carbon dioxide.
- Under normal conditions, oxygen produced during photosynthesis is released into the atmosphere and does not significantly affect the rate of photosynthesis.
- However, at high concentrations, oxygen can inhibit the rate of photosynthesis.

4. Mechanism of the Warburg Effect:
- The exact mechanism of the Warburg effect is not fully understood, but it is believed to involve the inhibition of certain enzymes involved in photosynthesis.
- One of the key enzymes affected by high oxygen concentrations is RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase), which plays a crucial role in the Calvin cycle.
- High oxygen levels can compete with carbon dioxide for the active site of RuBisCO, leading to decreased efficiency of carbon fixation and reduced photosynthetic rates.

5. Consequences of the Warburg Effect:
- The Warburg effect can have negative consequences for plant growth and productivity, especially under high oxygen conditions.
- Reduced photosynthetic rates can limit the production of glucose, which is essential for plant growth and energy storage.
- It can also lead to the accumulation of reactive oxygen species (ROS), which can damage plant cells and impair their functioning.

In conclusion, the decreased rate of photosynthesis at high concentrations of oxygen is referred to as the Warburg effect. This phenomenon involves the inhibition of key enzymes, such as RuBisCO, leading to reduced efficiency of carbon fixation and decreased photosynthetic rates.