With adjustments for inflation, the relative cost of welfare benefits

This sentence requires comparison of cost of the program now and in 1973. Since "less" is used here for comparison, the correct sentence must have "less .. than" structure. So B is out.
Now, it is the cost in 1973 that is being compared with the cost now, we should have "it did" before "in 1973". So A and E are out.

Now between C and D:
(c)resulting in the welfare program now costing about 3 billion real dollars less to implement than it did

C has two errrors
i) sentence implies that the "drop in cost of welfare benefits resulted in the welfare program". The drop did not result in the program itself, rather resulted in lower cost of the program. Here "now costing about..." is essentially modifying the noun "the welfare program". The word "costing" is not acting as a verb, rather as a -ing modifier. Hence the option C reads more like "the drop resulted in the welfare program", which has some other cost related features.
ii) Ideally "did" at the end represents past particle of the verb "cost". However, all we have is the -ing modifier "costing", which can not be considered as both modifier and verb in the same sentence.

In choice D, it is clear that "as a result it is the welfare program that now costs xyz than it did (cost) in 1973". since cost is used here as a verb next to "the welfare program", we can again use it as a verb as a replacement of "did".