The quality of work is not an issue here. Also answer E repeats the information from the task, thus it must be the correct choice.

It takes 4 weeks for a team of 5 professional Window Washers working regular full time hours to completely clean every window of the Empire State building. The building's owner demands that all the windows always be clean.Yet 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.

Which of the following statements as must be true on the basis of the statements above ?
A) If an empire state building window is to be kept clean, it must be cleaned by a professional cleaner.
B)The owner's demand for proper cleaning of all the windows will never be fulfilled.
C) If a team of 5 window washers cleans all the empire state building's windows in less than 4 weeks, some windows will not be properly cleaned.
D) In order to ensure that all of the Empire State Building's windows are clean, the owner must have his window washers work overtime.
E)Some Empire State Building windows must be cleaned more frequently than once every four weeks if they are to be kept clean.

Now let us go through the options one by one and check:

A-- we do not know because it is not mentioned in the argument that only professional cleaners can clean the windows. Incorrect.
B-- out of context and incorrect.
C-- we are not concerned here about windows being "properly cleaned". That is not the moot point. We are concerned about cleaning of windows. C digresses from the topic even if it may be true.
D-- again, out of context and incorrect.
E-- is very much consistent with what has been written in the passage and so must be true.

An important takeaway from this problem is that we are concerned with finding the correct answer, not the one that sounds true or right.

D is wrong -- Owner can have more people to work !!

IMO E

Since 5 workers will be able to clean the windows by doing overtime in ONE WEEK. So the only way the work will not be done is because some windows would require cleaning before four weeks are up.

Got rid of the other options by elimination.

Eliminated C because we are not Talking about cleaning windows completely.

must be true question should be answered by looking for restructured sentenses which give the argument or rather a part of it...

so when confused the one which best simulates the argument can lead you to right answer