The user is asking for the option that weakens the conclusion of the provided argument.

Argument Analysis
- Premise 1: Class grades are only A, B, or C.
- Premise 2: Average final score in 2012 was 5 points lower than in 2011.
- Conclusion: The percent of students who received 'C's was greater in 2012 than in 2011.

The Flaw: The conclusion assumes that the only way the average score could drop is if the proportion of the lowest grade ( ${ }^{\prime} C^{\prime}$ ) increased. However, the average score could drop if the proportion of the highest grade ('A') decreased, or if the proportion of the middle grade ('B') shifted closer to 'C' scores, or any combination of shifts in A's and B's.

Evaluating the Options
The correct option will show an alternative reason for the drop in the average score that does not require an increase in the percentage of 'C's.
- A) There was a greater number of students in the 2011 class than in the 2012 class.
- The total number of students does not affect the average score or the percentage breakdown of grades. (Irrelevant).
- B) The percent of students who received 'A's in 2011 was less than in 2012.
- If the percentage of A's increased in 2012, this would tend to increase the average score, making the 5-point drop harder to explain, and thus strengthening the need for a huge increase in 'C's. (Strengthens the conclusion, not weakens it).

- C) Five more students received 'A's in 2012, then in 2011.
- Similar to B, an increase in the raw number of A's makes the average drop harder to explain. (Strengthens the need for more C's).
- D) The same number of students received 'B's in 2011 as in 2012.
- This only addresses the raw number, not the percentage or the distribution of A's and C's. (Weakly relevant, but not the best explanation).
- E) The percent of students who received 'B's was greater in 2012 than in 2011.
- If the percentage of ' B 's increased, and we assume ' B ' scores are lower than ' A ' scores, this shift from 'A's to 'B's (a shift down the score distribution) could entirely account for the 5-point drop in the overall average score, even if the percentage of 'C's did not increase (or even decreased). This provides an alternative explanation for the average drop, suggesting the conclusion is not necessarily valid.

Conclusion: Option E provides an alternative mechanism (a shift from A's to B's) for the 5point average score drop, thus weakening the need to conclude that the percentage of C's must have increased.

The correct choice is $\(\mathbf{E}\)$.