What about A?
The reason they chose 7-9 month age range could be because of the obesity rate is different or lesser in children older than 9 months than the 7-9 months children and so this could be the basis of their research choosing that age range.

Children outside the age range of 7-9 months are out of scope for this question. We have to find out the assumptions researchers made in their research of obesity in that age group only. Whatever outliers that are not mentioned in the passage are out of scope.

This is a sampling assumption across time.

Premise

Researchers compared 42 average-weight and 47 obese infants, aged 7 to 9 months, with respect to current daily nutrient intake, ratio of formula or breast milk to solids in the diet, and maternal reliance on external feeding cues, such as time of day. Mothers completed a three-day food record at home before answering questions on current feeding practices.

Assumption

Pre-think: Three day feeding practices = feeding practices all the time

Only an assumption like this will lead the premise above to the conclusion below.

Choice from the list given - Choice B - In the months before the study, the feeding practices of the mothers in the study did not differ significantly from their feeding practices at the time of the study.

Conclusion

The researchers concluded from all of these data that, contrary to popular belief, the feeding practices of mothers of obese babies do not contribute significantly to their babies' obesity.