This argument reasons that because per-patient funding in the hospitals under consideration are equal yet there are differences in the mortality rates, that this is probably explained by differences in the quality of care patients receive from the hospital staff.
That is a bold conclusion! There could be many, many reasons of which why a particular hospital with equal per-patient funding would have a worse mortality rate without poorer quality of care. Perhaps the illnesses they treat are more severe, perhaps there are some environmental factors affecting the patients at that particular hospital. The list can go on and on.
Answer choice D doesn't weaken this argument because stating that approximately the same number of surgeries occur at the hospitals does not disprove or call into question the conclusion that mortality rates are higher due to poorer quality of care.
Answer choice B is the exact prediction mentioned above. And I did not look at the answer before writing that up. To me, that seems like an obvious hole in their reasoning that they didn't account for. These types of flaws recur. Worse success rate may be due to more difficult conditions. This is something that is worthwhile practicing. Both the practice of predicting answer choices, and the practice of reviewing answer choices to be able to predict future ones.
If one hospital treats more severely ill patients than another, than even with the best quality of care, it is still possible for that hospital to have a higher mortality rate.
I hope this helps. Please let me know if you have any other questions!