The response of different organisms to environmental rhythms of light and darkness is called:

Options

A. phototaxis
B. photoperiodism
C. phototropism
D. vernalisation

Explanation

To answer the question, "The response of different organisms to environmental rhythms of light and darkness is called:", we will analyze the options provided and identify the correct term.

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for the term that describes how different organisms respond to the cycles of light and darkness in their environment.

2. **Analyzing the Options**:
- **Phototaxis**: This term refers to the movement of an organism towards or away from light. For example, Euglena moves towards light, but this does not encompass the response to darkness.
- **Photoperiodism**: This term refers to the physiological reaction of organisms to the length of day or night. It is particularly relevant in flowering plants, where the duration of light (day) and darkness (night) affects flowering.
- **Phototropism**: This refers to the growth of plants towards light. It is about the direction of growth rather than a response to light and darkness rhythms.
- **Vernalization**: This is the process of inducing flowering in plants by exposing them to a prolonged period of cold temperatures, unrelated to light and darkness.

3. **Identifying the Correct Answer**: Among the options analyzed, **Photoperiodism** is the term that best describes the response of organisms to the environmental rhythms of light and darkness. It involves how plants and other organisms respond to the length of light and dark periods, influencing processes like flowering.

4. **Final Answer**: The correct answer is **Photoperiodism**.