What's the biggest obstacle to traveling by boat on the congo river?

This would be shallow rivers. They're the biggest obstacles by boat on the Congo River.
Hope this will help you!:)

C. shallow areas due to drought

Explanation:

The congo river is the second largest river of Africa and has a length of about 4,700km and has a basin area of about 4,014,500 km square. The congo river is a major  transport route carries 86 million tonnes of the suspended sediments towards the Atlantic ocean and about 6% in the bed load. But in the dry season the river is unable to carry and deposit the load and thus forms a bigger obstacle as major rapids hence not navigable.

The answer would be (c), mass extinctions

The answer for your problem would be a. 7