What characteristic gives the red river its name?

The name of the Red River derives from its color, which in turn comes from the fact that the river carries large quantities of red soil in flood periods. The river has a high salt content.  

The Red River is a major river in the southern United States of America. It received its named for the red-bed country of its watershed.

The south bank of the Red River formed part of the US–Mexico border from the Adams–Onís Treaty until the Texas Annexation and the Treaty of Guadalupe Hidalgo.

where is the diagram

i think the answer is b.