E looks correct.

Weight gain is different than Weight. Weight is expressed on a point of time. However, Weight gain is expressed over a period of time. For example, "three months before my weight was 60 Kg, however I gained 10 kg in last three months. "
Hence if child weight is 10 Kg, his weight is certainly less than US average. But we can not say anything about her weight gain.

The average normal infant born in the United States weights between 12 and 14 pounds at the age of three months. Therefore, if a three-month-old child weights only 10 pounds, its weight gain has been below the United States average.

Which of the following a flaw in the reasoning above?

A. Weight is only one measure of normal infant development.
B. Some three-month-old children weigh as much as 17 pounds.
C. It is possible for a normal child to weigh 10 pounds at birth.
D. The phrase "below average" does not necessarily mean insufficient.
E. Average weight gain is not the same as average weight.

Error Analysis:

The argument compares two completely different things. Average Weight Vs Average Weight Gain.

Average Weight - The sum of weight during a period divided by period( No. of months or years)
SO if u weigh 75,75,75 for 3 months your avg weight =75Kg.

Average Weight Gain - The increase in weight gained above the average over a period divided by period.
So if u weigh 75,76,77 for 3 months your avg weight gain = 1 Kg.

Only Option E, Corresponds to this error.

The average normal infant born in the United States weights between 12 and 14 pounds at the age of three months. Therefore, if a three-month-old child weights only 10 pounds, its weight gain has been below the United States average.

Which of the following a flaw in the reasoning above?

A. Weight is only one measure of normal infant development.
Very very tempting answer but are we concerned about the normal infant development?

B. Some three-month-old children weigh as much as 17 pounds.
The word some vs average clash with each other

C. It is possible for a normal child to weigh 10 pounds at birth.
For us anything is possible but we need to present a solid evidence for it.

D. The phrase "below average" does not necessarily mean insufficient.
We are again not concerned with insufficiency about it.

E. Average weight gain is not the same as average weight.
Bingo! The last sentence assumes that the weight gain is same as the average weight and therefore came to such flawed reasoning.

the passage is ambiguous
all it says is that

its weight gain has been below the United States average..
now a reader can infer this to be
its weight gain has been below the United States average weight gain
or
its weight gain has been below the United States average weight..

Isn't it?
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The flaw is that average weight does not give any indication about weight gain. Hence "E"

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