Rivers of drainage system. option - 1 Sao Francisco 2 amazon 3 subarnarekha 4 Uruguay

It is impossible to build a drainage system on the outside of the Sao Francisco river as buildings are built on the outside of the foundation.

The Subarnarekha is developing a small drainage system.

The Amazon basin is the largest drainage basin in the world.

The Urugay drainage system includes the greater part of southeastern Brazil, Paraguay, southeastern Bolivia and northern Argentina

So i would suggest putting the Amazon as it is the most common known and largest. Remember to use geographical terms!