Red tide is caused by rapid multiplication of

Options

A. Nostoc
B. Desmids
C. Diatoms
D. Gonyaulax

Answer

### Step-by-Step Solution:

1. **Understanding Red Tide**: Red tide is a phenomenon that occurs when there is a rapid increase in the population of certain microorganisms in water bodies, leading to a discoloration of the water.

2. **Identifying the Cause**: The primary cause of red tide is the rapid multiplication of specific types of algae, particularly dinoflagellates.

3. **Characteristics of Dinoflagellates**: Dinoflagellates are marine organisms that are photosynthetic. They can produce pigments that give the water a reddish hue, which is why the phenomenon is called "red tide."

4. **Analyzing the Options**:
- **Nostoc**: This is a type of cyanobacteria, not responsible for red tide.
- **Decimids**: These are a group of chrysophytes, which are also not responsible for red tide.
- **Diatoms**: Another group of algae, but they do not cause red tide.
- **Guanalux**: This is a type of dinoflagellate known for causing red tide due to its red pigments.

5. **Conclusion**: Based on the characteristics and the options provided, the correct answer is **Guanalux** (Option D), as it is the organism responsible for the red coloration in water during red tide events.