Identify the poor word choice and replace it with a word of your own. my endurance with the task was waning.

In this sentence, the poor word choice would be waning. It is not very common in this context, you could use a similar word, with the same meaning, and that "feels more comfortable" with the sentence, as it referred to a casual subject and, in my opinion, it has to be easy to read.

You could use words such as fading, reducing, vanishing, disappearing.

ich statement best shows a problem in travels with charley?

one sharp difficulty presented itself. in the intervening twenty-five years my name had become reasonably well known.

this being so, my trip demanded that i leave my name and my identity at home.

i had to go alone and i had to be self-contained, a kind of casual turtle carrying his house on his back.

with all this in mind i wrote to the head office of a great corporation which manufactures trucks.

your answer is c) experiments with water vapor showed that elements combine in specific proportions.