Two species cannot occupy same :

Options

A. Habitat
B. Ecological Niche
C. Size
D. Photoperiod requirement

Answer

To solve the question "Two species cannot occupy same:", we need to analyze the options provided and understand the underlying biological principles.

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about a specific aspect of ecology where two species cannot coexist. This relates to how species interact with their environment and each other.

2. **Reviewing the Options**: The options given are:
- Habitat
- Ecological niche
- Size
- Photoperiod requirement

3. **Analyzing Each Option**:
- **Habitat**: Many species can occupy the same habitat. For example, different bird species can live in the same forest.
- **Ecological Niche**: This refers to the role and position a species has in its environment, including its habitat, resource use, and interactions with other species. According to the competitive exclusion principle, two species cannot occupy the same niche in the same habitat and coexist stably.
- **Size**: Different species can have the same size. Size does not restrict species from coexisting.
- **Photoperiod Requirement**: Different species can have similar or the same photoperiod requirements and still coexist.

4. **Applying the Competitive Exclusion Principle**: This principle states that if two species compete for the same resources and occupy the same niche, one will outcompete the other, leading to the exclusion of one species.

5. **Conclusion**: Based on the analysis, the correct answer is **Ecological Niche**. This is because two species cannot occupy the same ecological niche without competing for the same resources, which would lead to one species being excluded.

### Final Answer:
**Ecological Niche**

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