A= doesnt talk about water power usage
B- doesnt explain our question
C- not related.
D- it gives us alternative that they used more traditional sources but doesnt answer why they didnt use water power.
E- answers our question.

I am not very happy with this question, please check the source once. The argument says large cities. In choice E - We have to assume here that large city is heavily populated. ( why do we need to do that??) This makes this choice vague.

This makes it all the more striking that the Romans made do without water power in dominated by large cities

i have not understood this line ... can anyone rewrite this ??

"make do" means to manage.
for example: the prisoners made do on 1 gallon of water for 10 days.

Guys dnt worry much on this question as this is LSAT question . I found it tricky hence posted . Actually the catch here is that all options are trying to tell about some alternate strategy but none except E is telling us why Romans took such action and solve the paradox .

Doubt - my doubt here is;
Frankly none of the options fit in properly. Option E "In heavily populated areas the introduction of water power would have been certain to cause social unrest by depriving large numbers of people of their livelihood".
Now this option assumes that large cities are heavily populated. A small city can be heavily populated also, causing the city to be devastated by the wrath of the river.
I chose A for the fact that it states that The ancient Romans were adept at constructing and maintaining aqueducts that could carry quantities of water sufficient to supply large cities over considerable distances. Now the water is sufficient just for communal purposes, not for generating energy. Option E is poorly worded.

Solution - If you read the question properly, you would notice that the question is about the usage of water power. Option A merely states that water could be transported to large cities. However, we are not concerned about that. We want to know why water power was not used?
The correct reason was brought up by E which gives a sufficient reason. Hence, I believe that considering the options available, the best choice is E and we have to assume that large cities are highly populated.

Experts - can we have your thoughts on this explanation?