The correct answer is option 'C' - Magascolex.

Explanation:
The south Indian earthworm is scientifically known as Magascolex. It is a type of earthworm found in the southern region of India.

Here is a detailed explanation of the options given:

a) Ponoblada: This is not the correct zoological name for the south Indian earthworm. It is not a recognized scientific name for any species.

b) Lumbricus: This is a genus of earthworms that includes several species found in different parts of the world, but it is not the specific zoological name for the south Indian earthworm.

c) Magascolex: This is the correct zoological name for the south Indian earthworm. It belongs to the family Megascolecidae and is commonly found in the southern states of India.

d) Pheretima: This is another genus of earthworms that includes several species found in various parts of the world. However, it is not the specific zoological name for the south Indian earthworm.

Therefore, the correct answer is option 'C' - Magascolex.

To summarize:
- The south Indian earthworm is scientifically known as Magascolex.
- Option 'C' is the correct answer.
- Option 'C' represents the specific zoological name for the south Indian earthworm, while the other options are either incorrect or represent different genera of earthworms.