The consumer price index is a measure that detects monthly changes in

Let's break down our passage.
1. index detects monthly changes in prices.
2. payment of benefits is based on index.
3. (but) index does not consider technology which will reduce cost
4. (therefore) benefits are sometimes greater than necessary.

As the logic between the statements is clear, we'll first try to infer our answer.
This is a Precise approach.

The main logical claim is (3)-->(4), which states that better technology means lower production costs and then infers that this means that the benefits are inflated. For this link to be true, we would need another statement, that explains why the lowered production costs would cause lower prices without changing the index.
Looking at our answers, (E) is the only statement which touches on the production --> pricing gap.
Let's take a closer look at the other options, just to be sure.

(A) fails to consider the possibility that there are years in which there is no change in the consumer price index But what about years in which there is?
(B) fails to make explicit which goods and services are included in the consumer price index This is very general and doesn't explain why the main claim, that the benefits are greater than necessary, is false.
(C) presumes, without providing warrant, that retirement benefits are not generally used to purchase unusual goods This weakens the argument by claiming that the index isn't representative of what benefits are actually used for. However, it is a bit roundabout and is missing a link which states that unusual good aren't reflected by the index. Moreover, it doesn't address the production --> pricing gap
(D) uncritically draws an inference from what has been true in the past to what will be true in the future This is far too generic to be an answer (and does not actually address any of the points)
(E) makes an irrelevant shift from discussing retail prices to discussing production costs As explained above, this is the weakest link.

(E) is our best option.