Red drop is

Options

A. Drop in oxygen yield
B. Drop in quantum yield
C. Drop in organic yield
D. Drop in photosynthetic yield.

Answer

### Step-by-Step Solution:

1. **Understanding the Concept of Red Drop**:
- The term "red drop" refers to a phenomenon observed in photosynthesis where there is a significant decrease in the quantum yield of oxygen production when light wavelengths exceed 680 nanometers.

2. **Defining Quantum Yield**:
- Quantum yield is defined as the number of oxygen molecules released per quantum (or unit) of light absorbed during the process of photosynthesis. It is a measure of the efficiency of photosynthesis.

3. **Identifying the Characteristics of Red Drop**:
- The red drop effect is characterized by a sharp decrease in quantum yield at wavelengths greater than 680 nanometers. This means that as the wavelength of light increases beyond this point, the efficiency of photosynthesis in terms of oxygen production drops significantly.

4. **Context of the Red Drop Effect**:
- This effect is primarily observed in green plants, which are the main organisms performing photosynthesis. The decrease in quantum yield indicates that light in the red region of the spectrum (above 680 nm) is less effective for photosynthesis compared to light in the blue and green regions.

5. **Conclusion**:
- Based on the definition and characteristics of the red drop effect, we conclude that the red drop is a sharp decrease in quantum yield, specifically in the context of photosynthesis in green plants.

### Final Answer:
The red drop is a sharp decrease in quantum yield at wavelengths greater than 680 nanometers.