Answer: Option D
Hydel power potential refers to the potential of a river to generate hydroelectricity. It is determined by the following factors:

• Water discharge: The amount of water that flows through a river determines its power potential. The higher the water discharge, the greater the potential for hydroelectricity generation.
• Head: The head is the vertical distance between the water level upstream and downstream of the hydropower plant. The greater the head, the higher the potential for power generation.
• Catchment area: The catchment area of a river determines the amount of water that can be stored and used for power generation. A larger catchment area means more water can be stored, and hence, more power can be generated.

Now, let's see how the West flowing rivers of South India fare on these factors as compared to the other options:
A. Brahmaputra: Although Brahmaputra has a high water discharge, it has a relatively low head, and its catchment area is smaller than that of West flowing rivers of South India.
B. Ganga basin: The Ganga basin has a large catchment area and a high water discharge, but its head is lower than that of the West flowing rivers of South India.
C. East flowing rivers of South India: The East flowing rivers of South India have a relatively small catchment area and a low water discharge as compared to the West flowing rivers of South India.
D. West flowing rivers of South India: The West flowing rivers of South India have a combination of high water discharge, large catchment area, and high head, making them the most suitable for hydroelectricity generation.
Therefore, the correct answer is Option D, i.e., West flowing rivers of South India, and the factors mentioned above explain why they have the highest hydel power potential.