When a species becomes extinct, the plant and animal species associated with it in an obligatory way also become extinct. This phenomenon is referred to as
(a) fragmentation
(b) alien species invasion
(c) over-exploitation
(d) co-extinction

Options

A. fragmentation
B. alien species invasion
C. over-exploitation
D. co-extinction

Explanation

To solve the question regarding the phenomenon where the extinction of one species leads to the extinction of associated species, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question states that when a species becomes extinct, other plant and animal species that are associated with it in an obligatory way also become extinct. We need to identify the term that describes this phenomenon.

2. **Analyze the Options**:
- **Option (a) Fragmentation**: This term refers to the breaking apart of habitats or ecosystems, leading to isolated populations, but it does not specifically relate to the extinction of associated species.
- **Option (b) Alien Species Invasion**: This refers to non-native species that invade an ecosystem, potentially disrupting local species, but it does not describe the extinction relationship.
- **Option (c) Over-exploitation**: This term refers to the excessive use of species, leading to their decline or extinction, but it does not imply a direct relationship of mutual dependence.
- **Option (d) Co-extinction**: This term specifically refers to the phenomenon where the extinction of one species leads to the extinction of another species that is dependent on it.

3. **Identify the Correct Answer**: Based on the analysis, the correct answer is **(d) co-extinction**. This term accurately describes the relationship where the extinction of one species results in the extinction of another species that is obligately associated with it.

### Final Answer:
The correct answer is **(d) co-extinction**.

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