I'd pick D. But I don't like this question (I even thought there's a typo there). See below why.
Photovoltaic power plants produce electricity from sunlight. As a result of astonishing recent technological advances, the cost of producing electric power at photovoltaic power plants, allowing for both construction and operating costs, is one-tenth of what it was 20 years ago, whereas the corresponding cost for traditional plants, which burn fossil fuels, has increased. Thus, photovoltaic power plants offer a less expensive approach to meeting demand for electricity than do traditional power plants.
The conclusion of the argument is properly drawn if which one of the following is assumed?
The argument gives us info about the increases in costs
. However, we do not know the relation between costs themselves
. At the same time, the argument makes a conclusion precisely about the relation between costs
So, we need additional assumption that will help us to evaluate
whether ‘Photovoltaic plants costs < Traditional plants costs’ is indeed true.
Here I think little calculation is needed:
Photovoltaic plants costs - reduced by 10 times
Traditional plants costs – increased (by unspecified amount
Thus, 20 year ago Photovoltaic plants costs should be no more than 10 times
of Traditional plants costs. Otherwise, we can't guarantee that the conclusion is true.
Now it’s time to look what we have:
(A) The cost of producing electric power at traditional plants has increased over the past 20 years.
(B) Twenty years ago, traditional power plants were producing 10 times more electric power than were photovoltaic plants.
(C) None of the recent technological advances in producing electric power at photovoltaic plants can be applied to producing power at traditional plants.
(D) Twenty years ago, the cost of producing electric power at photovoltaic plants was less than 20 times the cost of producing power at traditional plants.
(E) The cost of producing electric power at photovoltaic plants is expected to decrease further, while the cost of producing power at traditional plants is not expected to decrease
The closest to what we need is D.
However, there’s a tricky moment here. D by itself is not enough
to draw a proper conclusion. It gives us a weaker condition than we need: ‘less than 20 times’, while we need ‘less than 10 times’. However, we could say that D is a part of the assumption
that is needed to draw a conclusion (since is ‘less than 10 times’ is true, D will also be true).
Overall, I think we have a good enough reason to choose D. But, honestly, I think that this CR just goes too far. I don’t believe that on GMAT we will be required to do all this reasoning.