Over the last three years, the city of Las Vegas has grown in populati

Good question. The city council has "proposed" a moratorium on water consuming golf courses "starting in two years"

Hence, the decline in water uses in last three years is not its effect. It is clear from the words in bold.

Questions says "Each of the following, if true, could help to explain the simultaneous decline in water usage and increase in population in Las Vegas" , so doesn't it mean that apart from one choice among the given 5, the rest should somehow indicate decline in water usage and increase in population happening simultaneously ?

Hi nick
You misunderstood the logic behind "resolve the paradox" questions a little bit.
(1) In the stimulus we have facts that create a paradox. The paradox only exists in the context of the facts.
(2) The facts always exists simultaneously

Back to your question.
The "resolve the paradox" question stem is: Each of the following, if true, could help to explain the paradox in the context provided, except:
==> Each answer, if true can explain the paradox (why water usage declined) in the context - simultaneous decline in water usage and increase in population in Las Vegas.

The answers themselves indicate the context already. If the answer does NOT attach the context in it, the paradox is meaningless. Thus, the answers do not need to indicate both water usage and population increase. That's redundant. We just need to find a gap between two simultaneous facts.

Hope it's clear.

Over the last three years, the city of Las Vegas has grown in population at a rate of 6 percent per year. Over the same period of time, however, water usage inside the city of Las Vegas is down almost 15 percent.

Each of the following, if true, could help to explain the simultaneous decline in water usage and increase in population in Las Vegas, EXCEPT:

A) Due to drought awareness campaigns, the average citizen in Las Vegas now uses almost 30 percent less water than he or she did three years ago.
B) Two years ago, the city instituted a popular new incentive program for citizens to tear out high-water-consuming lawns and replace them with low-water-consuming desert landscaping.
C) The city council has proposed a moratorium on the construction of high-water-consuming golf courses starting in two years.
D) Three years ago, a mining company that consumed vast amounts of water operated within the city limits; now that company is out of business.
E) The mayor ordered that all of the city's hundreds of fountains be shut off, considerably reducing water use due to evaporation.

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Nice,easy C.C does not explain why the water consumption came down.The rest of the options do.

OFFICIAL EXPLANATION

This is an Explain EXCEPT question. Expect there to be a paradox or apparent contradiction in the stimulus and for the four wrong choices to explain how both parts of the apparent contradiction can be true at the same time. The correct answer will not explain it and may even deepen the mystery.

The two parts of the apparent contradiction in an Explain question are often separated by a contrast word, such as "but," "yet," or, in this case, "however." On the one hand, the population of Las Vegas has grown at a rate of 6 percent per year. On the other hand, water usage is down almost 15 percent.

Explain questions are often not predictable. Typically, there is just no way to know what the GMAT will come up with to explain the situation. Just be clear on what the two parts of the apparent contradiction are and expect to find four choices (in this EXCEPT question) that explain how both parts can be true at the same time. Here, the four wrong choices will explain how Las Vegas water usage could be down almost 15 percent even though the population has grown.

(C) refers to a plan that might help to save water in the future by blocking the construction of new golf courses. However, this plan would have had no effect on the last three years, so it cannot help to explain the paradox. Thus, (C) is correct.

(A) would be able to explain the whole situation by itself. The lowering of water consumption per person by such a dramatic amount would allow for a 15 percent drop in water usage, despite the fact that more people now live in the city.

(B), if true, could also explain a lower amount of water used in the last two years. If lawns consume significant amounts of water and there are now fewer of them, then one would expect a decline in water use.

(D) and (E) also refer to other significant uses of water that have been eliminated in the three-year period discussed in the stimulus - in the first case, the closing of a water-consuming mine and, in the second case, the shutting off of hundreds of municipal fountains.

On using POE, we can find Option C is the only one which do not talk about decline in water usage.

Option C says City Council has proposed to start after two years, but we don't know whether it is implemented or not.