An interaction favourable to both the population , but not obligatory to either is :

Options

A. Proto-cooperation
B. Mutuaslim
C. Commensalism
D. Parasitism

Answer

To answer the question regarding an interaction that is favorable to both populations but not obligatory to either, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Types of Interactions**: Start by recognizing the different types of ecological interactions that exist between organisms. The main types include mutualism, commensalism, parasitism, and proto-cooperation.

2. **Defining Mutualism**: Mutualism is a type of interaction where both organisms benefit from each other, and they often live in the same habitat. An example is the relationship between fungi and algae in lichens.

3. **Defining Commensalism**: In commensalism, one organism benefits while the other is neither helped nor harmed. An example is the remora fish that attaches to sharks for protection.

4. **Defining Parasitism**: Parasitism is a negative interaction where one organism (the parasite) benefits at the expense of the other (the host). This is clearly not the type of interaction we are looking for.

5. **Defining Proto-cooperation**: Proto-cooperation is a type of interaction where both organisms benefit from each other, but the interaction is not obligatory for their survival. An example is birds that eat ectoparasites off cattle. The birds benefit by getting food, while the cattle benefit from the removal of parasites, but neither is dependent on the other for survival.

6. **Conclusion**: Based on the definitions and examples, the interaction that is favorable to both populations but not obligatory to either is **proto-cooperation**.

### Final Answer:
The correct answer is **proto-cooperation**.

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