Funny enough, at first glance, I got the same impression as you did. But now that I read it again, it is clear that the argument says that countries with socialized medicine has less infant mortality than countries that rely on private medicine and this therefore means that socialized medicine is technologically superior.
Answer choice A states that low infant mortality rate in countries with socialized medicine is due to access to medical care. This weakens the argument by showing that it is not the technological superiority of socialized medicine that is the cause, but rather access.
I hope this helps. Please let me know if you have any other questions or would like me to elaborate further.