Hills reaction completes in

Options

A. presence of light
B. absence of light
C. absence of water
D. all the above

Answer

### Step-by-Step Solution:

1. **Understanding Hill's Reaction**:
- Hill's reaction is a process named after the scientist who discovered it. It describes a specific biochemical reaction that occurs during photosynthesis.

2. **Nature of the Reaction**:
- It is a light-driven reaction, meaning it requires light to occur. This is a crucial point because without light, the reaction cannot take place.

3. **Electron Transfer**:
- The reaction involves the transfer of electrons. Specifically, electrons are transferred from water molecules to Hill reagents. This process is essential for the photosynthetic pathway.

4. **Potential Gradient**:
- The transfer of electrons occurs against a potential gradient, which indicates that energy is required to move the electrons from a region of lower potential to a region of higher potential.

5. **Conditions for Hill's Reaction**:
- For Hill's reaction to occur, two main conditions must be satisfied:
- **Presence of Light**: The reaction can only take place in the presence of light.
- **Presence of Water**: Water is necessary as it is the source of electrons.

6. **Evaluating Options**:
- Given the options:
- Presence of light: This is correct as Hill's reaction occurs in light.
- Absence of light: This is incorrect as the reaction cannot occur without light.
- Absence of water: This is also incorrect since water is essential for the electron transfer.
- All of the above: This option is incorrect as well.

7. **Conclusion**:
- The correct answer to the question "Hill's reaction completes in" is **presence of light**.