Which one of the following fish is being illegally introduced for aquaculture purposes and is posing a threat to the indigenous catfishes of Indian rivers?
(a) Clarias gariepinus
(b) Nile perch
(c) climbing perch
(d) Protopterus

Options

A. Clarias gariepinus
B. Nile perch
C. climbing perch
D. Protopterus

Explanation

To solve the question regarding which fish is illegally introduced for aquaculture and threatens indigenous catfishes of Indian rivers, follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question asks for a specific fish species that has been illegally introduced into Indian rivers for aquaculture and poses a threat to local catfish species.

2. **Analyze the Options**:
- (a) Clarias gariepinus
- (b) Nile perch
- (c) Climbing perch
- (d) Protopterus

3. **Identify the Fish**:
- **Clarias gariepinus**: Known as the African catfish, it has been introduced into various regions, including India, for aquaculture purposes. It is known to outcompete native species and can pose a significant threat to indigenous catfishes.
- **Nile perch**: While it is an invasive species, it primarily affects fish populations in Lake Victoria and does not threaten Indian river catfishes.
- **Climbing perch**: This fish is also invasive but is not specifically known for threatening indigenous catfishes in India.
- **Protopterus**: This is a type of lungfish and does not have a significant impact on Indian catfish populations.

4. **Conclusion**: After evaluating the options, it is clear that **Clarias gariepinus** is the fish that has been illegally introduced for aquaculture and poses a threat to indigenous catfishes in Indian rivers.

5. **Final Answer**: The correct option is (a) Clarias gariepinus.