Our original argument precedes by stating that the obesity epidemic is a myth. The author bases this conclusion on the fact that the average increase in wieght was only 1 pound- which is not substantial- so the proportion of obese children must not have changed substantially.
This argument is flawed as makes a claim about the proportion of obese chilren based only on the average increase in weight. An average tells us nothing about the distribution. The author falsely assumes that every child gained exactly one pound, but it is equally possible that half of children gained two points, or ten percent of children gained ten pounds.
A is a different flaw- the original argument had nothing about people's beliefs.
The argument in B exhibits this exact same flaw. The author mistakenly assumes that a change in an average is applied equally to every member of the group in question.
C is a different structure, as the conclusion is that there was an increase, not that there was not an increase.
D is a different flaw, as the argument is not based on a misunderstanding of averages.
E is a different structure, as it uses a change in proportion to make a conclusion about a change in an average, rather than the opposite.