Select the most appropriate option to fill in the blank.
The Nile is the ________ river of the world.

Correct option is D

The correct option to fill in the blank is (d) longest.
When comparing more than two things or referring to the highest degree, we use the superlative form.
Meanings of other options:
· Long: of great length.
· Longer: comparative form of "long."
· More longer: incorrect comparative form.