good question gmatmachoman

This is a causual reasoning statement.

the conclusion is Adequate prenatal care -> decreases the risk of low birth weight babies
(cause) (effect)

ways to weaken a causual reasoning statement:

1. show that an alternative cause exists.
2. cause occurs but effect does not occur
3. cause does not occur but effect occurs
4. E -> C
5. Data invalidation.

Now lets look at options:

(A) The hospital records indicate that many babies that are born with normal birth weights are born to mothers who had inadequate prenatal care.[ this option talks about normal babies born to mothers receiving inadequate pre natal care, but it does not affect the conclusion that adequate pre natal care decreases the risk of low birth weight babies. Eliminated!]

(B) Mothers giving birth prematurely are routinely classified by hospitals as having received inadequate prenatal care when the record of that care is not available. [Data invalidation. correct answer!]

(C) The hospital records indicate that low birth weight babies were routinely classified as having been born prematurely. [ though data invalidation exists over here, it does not touch upon the issue of pre natal care. hence eliminated!]

(D) Some babies not born prematurely, whose mothers received adequate prenatal care, have low birth weights. [ this is a restatement of the stimulus. the stimulus says "mothers who had received adequate prenatal care were less likely to have
low birth weight babies ".What does it mean less likely, it means that there are some mothers who received adequate prenatal care, and have given birth to low birth weight babies. Eliminated!]

(E) Women who receive adequate prenatal care are less likely to give birth prematurely than are women who do not receive adequate prenatal care. [ this is infact a strengthener. it strengthens the conclusion that adequate pre natal care ecreases the risk of low birth weight babies. Eliminated!]

Conclusion : Adequate prenatal care significantly decreases the risk of low birth weight babies

Since there is statistical data involved in the stimulus, the easiest way to weaken the argument is to show a flaw in the data.

(A) The hospital records indicate that many babies that are born with normal birth weights are born to mothers who had inadequate prenatal care.
Incorrect : Even if many babies are born with normal birth weights to mothers who had received inadequate prenatal care it does not decrease the risk of low birth weight babies

(B) Mothers giving birth prematurely are routinely classified by hospitals as having received inadequate prenatal care when the record of that care is not available.
Correct : This attacks the conclusion by stating that when record is not available it is classified as Mothers of premature babies received inadequate prenatal care.

(C) The hospital records indicate that low birth weight babies were routinely classified as having been born prematurely.
Incorrect : Irrelevant. This does nothing to undermine the conclusion

(D) Some babies not born prematurely, whose mothers received adequate prenatal care, have low birth weights.
Incorrect : Irrelevant.

(E) Women who receive adequate prenatal care are less likely to give birth prematurely than are women who do not receive adequate prenatal care.
Incorrect : Irrelevant. Does nothing to hurt the conclusion that adequate prenatal care significantly decreases the risk of low birth weight babies