Which movement occurs due to external stimulus ?

Options

A. Tropic
B. Nastic
C. Tactic
D. All the above.

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks which type of movement in plants occurs due to external stimuli.

2. **Identifying the Types of Movements**: We have three types of movements to consider:
- Tropic movement
- Gnostic movement
- Tactic movement

3. **Defining Tropic Movement**:
- Tropic movements are growth movements that occur in response to an external stimulus.
- They can be directional, meaning the plant grows towards or away from the stimulus (e.g., phototropism - growth towards light).

4. **Defining Gnostic Movement**:
- Gnostic movements are non-directional movements that also occur in response to an external stimulus.
- These movements do not involve growth but are reactions to stimuli (e.g., closing of leaves in response to touch).

5. **Defining Tactic Movement**:
- Tactic movements are movements that occur due to external stimuli such as light, chemicals, and temperature.
- Examples include:
- Phototactic movement (response to light)
- Chemotactic movement (response to chemicals)
- Thermotactic movement (response to temperature).

6. **Conclusion**: Since all three types of movements (tropic, gnostic, and tactic) occur due to external stimuli, the correct answer to the question is **D: all of the above**.