This answer choice gives us an additional correlation between smaller family size and likelihood of allergies, but it doesn't bridge the gap between family size and exposure to germs. Remember that to strengthen this argument, we're looking to strengthen the idea that being in a larger family exposes infants to more germs than being in a smaller family, and so reduces risk of allergies. This answer choice is broad enough that the change in allergies could be explained by other factors than germ exposure within the family, such as environmental changes over the last century. So this doesn't strengthen the argument.

This answer choice weakens the author’s argument. It suggests an alternative cause for the higher rates of allergies among children in smaller families: not exposure to germs, but the sorts of foods they eat.

This is completely irrelevant. The conclusion is about connecting family size, germ exposure, and developing allergies, not about the long-term consequences of some allergies and some germs.

This answer choice might seem to be implying that allergies are hereditary--but notice that the increased likelihood of allergies isn't necessarily blamed on genetics. It could also be environmental, given that children will likely grow up in similar surroundings to their parents. Either way, we have no idea how common such families are, and regardless of whether the risk of allergies is higher from the start due to genetics or environment, we want to know whether increased germ exposure in infancy will reduce that risk. This answer choice doesn't tell us anything about that.

Even though this answer choice doesn't specifically state the assumption at the heart of the argument--that children in large families are likely exposed to more germs--it strengthens the argument by showing a parallel case where the same assumption applies. Even children from small families, if exposed to other children in early infancy, are less likely to develop allergies than children exposed later. By giving us another example of a positive correlation between being exposed to more children, thus being exposed to more germs, and reduced risk of developing allergies, this answer choice strengthens the argument.