Which of the following movements in plants is related to the changes in the auxin level?

Options

A. Movement of shoot towards source of light
B. Nyctinasty
C. Movement of sunflower towards sun
D. All of the above

Answer

To solve the question "Which of the following movements in plants is related to the changes in the auxin level?", we will analyze each option in relation to auxin's role in plant movements.

### Step-by-Step Solution:

1. **Understanding Auxin**:
- Auxin is a type of plant hormone that plays a crucial role in regulating plant growth. It promotes stem elongation and inhibits the growth of lateral buds.

2. **Analyzing the Options**:
- **Option 1: Movement of the shoot towards the source of light**:
- This movement is known as phototropism. When light is directed towards one side of the plant, auxin accumulates on the shaded side. This causes the cells on that side to elongate more than those on the light-exposed side, resulting in the bending of the shoot towards the light. Thus, this movement is indeed related to changes in auxin levels.

- **Option 2: Nictinasty**:
- Nictinasty refers to the movements of plants in response to the onset of darkness, such as the closing of flowers at night. This movement is not primarily driven by auxin, but rather by other factors such as circadian rhythms. Therefore, this option is incorrect.

- **Option 3: Movement of the sunflower towards the sun**:
- This is another example of phototropism, similar to option 1. While sunflowers do exhibit this movement, it is primarily a response to light rather than a direct result of auxin changes. However, it is still related to auxin as it involves the same mechanism of differential growth. But since the question asks for a direct relation, we will consider this option as less relevant compared to option 1.

- **Option 4: All of the above**:
- Since options 2 and 3 are not directly related to auxin changes, this option cannot be correct.

3. **Conclusion**:
- Based on the analysis, the movement of the shoot towards the source of light (Option 1) is the most directly related to changes in auxin levels. Therefore, the correct answer is **Option 1**.

### Final Answer:
**Option 1: Movement of the shoot towards the source of light.**