The sentence that contains an error in word usage is:
Working for my aunt, I had to walk Buster everyday, and bathe him once a week.
The correct version of the sentence should be:
Working for my aunt, I had to walk Buster every day, and bathe him once a week.
Explanation:
1. In the original sentence, "everyday" is used incorrectly. "Everyday" is an adjective that means common or ordinary, while "every day" should be used to indicate something that happens each day.
2. By changing "everyday" to "every day," the sentence becomes grammatically correct and conveys the intended meaning that walking Buster happens daily, not that it is common or ordinary.