Question

fix the one word that is used incorrectly.

only one out of every ten climbers finishes the five- to six-day assent
of mount kilimanjaro, which, at 19,340 feet, is the tallest freestanding
mountain in the world.

Explanation:

The word "assent" means agreement, which does not fit the context of climbing a mountain. The correct word for a climb up a mountain is "ascent".

Answer:

Replace "assent" with "ascent". The corrected sentence is: Only one out of every ten climbers finishes the five- to six-day ascent of Mount Kilimanjaro, which, at 19,340 feet, is the tallest freestanding mountain in the world.