B. The Amazon River system
Explanation:
The Amazon River flows through South America and we can see this by looking at a map. It dominates the North and spreads down and across most of South America.
We can also look at the other options, and see how they don't apply:
A. Easter Island
-> This is an island off of Chile
-> Does not really "dominate" South America as a whole
C. The Atacama Desert
-> The Atacama Desert is only in Northern Chile
-> Does not really "dominate" South America as a whole
D. The Brazilian coast
-> While the Brazilian coast is quite large, again, it is only along Brazil.
-> Does not really "dominate" South America as a whole
The image is from study.com.