This is a must be true question type, which means that you need to choose the answer type that can be proven true from the information in the passage. Usually an answer for a must be true passage is based on a premise or a combination of premises.

Choice E: Choice E is about windows. It is based on the first and last line of the argument. The last line states that
"even if the 5 washers work consistently throughout their regular work week, they will not be able to finish cleaning all the windows before some windows will need cleaning" This implies that some windows need to be cleaned more frequently.
When you combine this information with the information in the first line (which implies that it takes 4 weeks to clean all the windows) It can be inferred that at least some windows will need cleaning more frequently than once every 4 weeks.

Choice C: Choice C talks about the quality of work, something that we cannot infer about from the passage. It states that if the cleaners finish their work in less than 4 weeks then they will not clean the windows properly. This cannot be implied from the passage since cleaners take 4 weeks if they work their regular hours. There is a chance that the cleaners may be able to clean windows properly and yet finish the work in less than 4 weeks if they work overtime. Moreover, there is no indication in the passage that implies that their (Cleaner's) quality of work may be impacted if they rush through their work.