Nematocysts are activated by

Options

A. Water
B. Touch
C. Brain
D. None of the above.

Answer

### Step-by-Step Solution:

1. **Understanding Nematocysts**: Nematocysts are specialized cells found in members of the phylum Cnidaria (also known as Coelenterata). They are commonly referred to as stinging cells and are primarily used for capturing prey, anchoring, and defense.

2. **Structure of Nematocysts**: Each nematocyst contains a thread-like structure that is coiled inside the cell. This structure is under high osmotic pressure, which allows it to be expelled rapidly when triggered.

3. **Activation Mechanism**: The activation of nematocysts occurs in response to stimuli. These stimuli can be mechanical (such as touch) or chemical in nature.

4. **Types of Stimuli**:
- **Mechanical Stimuli**: This includes physical contact or touch, which is one of the primary triggers for nematocyst activation.
- **Chemical Stimuli**: Certain chemicals can also activate nematocysts, but in the context of the question, we focus on mechanical stimuli.

5. **Role of Nidosil**: The nidosil is a structure that acts as a trigger for the nematocyst. When stimulated, it causes the nematocyst to release its contents, including neurotoxins, which can immobilize prey or deter predators.

6. **Conclusion**: Based on the information provided, the correct answer to the question "Nematocysts are activated by" is **touch**.

### Final Answer:
Nematocysts are activated by **touch**.