Nice catch Vinzy.
I have a slightly different take on this question, and it has to do with taking what has been written very, very literally--a useful habit to cultivate on this test.
There was a red flag suggested by the phrase "believe they are more productive" that made me watch out for answer choices that had to do with perception. C does that--90% of 25% of the employees discussed (or 22.5% of the employees who work 8 hours a day but sometimes work at home) have a belief about their productivity that doesn't mesh with the difference in the number of hours actually worked.
If you saw that right away, awesome. If not, our old friend POE will get us there:
(A) Can we make this statement false? What's the extreme case? We can only use the 25% figure to make our calculation (since the 90% figure is about BELIEF IN PRODUCTIVITY rather than house worked).
Let's say we have 100 people. We want to see if we can make the average number of hours worked at home equal to or greater than the 8 hours worked at the office :
25 people work .9999999 hours at home
75 people work 24 hours at home
->Average--more than 18 hours per person worked at home
We know that A does not *have* to be true, so ELIMINATE
(B) "believe they are more productive" is different from "are more productive," and "hours worked" is different from "productive"--ELIMINATE
(C) Correct! 22.5% of the employees discussed must not define productivity just in terms of hours worked if they work fewer hours but believe they are more productive. 22.5% is more than 15%, so this answer looks good. Hold onto it.
(D) Nowhere did it say the amount of work at home accomplished was the same at both places--we just have statements about belief and hours worked--ELIMINATE
(E) We don't know whether they are or are not more productive (hours worked does not necessarily equal productivity, as many cubicle-dwelling facebook-procrastinators can tell you)--ELIMINATE
That leaves C.
IMPORTANT--The calculation in A concretely proves that A is not the answer, but I'm not suggesting that you do that calculation right away. In fact, a better strategy is to "defer" on answers that you think are possible but may require more work than you have time for, and only do that work if it's *less* work than the alternatives. If you got rid of B, D, and E, the calculation required for choice C is much quicker than the calculation required for A-- and once you know concretely that C MUST be true, you can choose it with confidence.