The key issue in this question is PARALLELISM.
Answer a is incorrect since the two phrases compared are different parts of speech. Also it sounds as if "the welfare program" resulted from the drop in costs. The correct structure should be "the welfare program's now costing"
Answer b - incorrect. The wording is exactly the same but "than" was replaced with "as." However, the structure is still wrong.
Answer c also contains the same mistake although the part of the sentence following THAN was corrected.
"resulting in the welfare program now costing about 3 billion real dollars less to implement than it did [cost]"
Answer d is correct.
Answer e is incorrect since it compares a clause with a prepositional phrase.