The first time one of my students showed me this one, I picked the wrong answer. After reviewing it, I see where the error came from.
The conclusion is that leaving the Pendleton Act unchanged is the best way to keep American gov't service "anchored to competence rather than corruption" after the year 2000. Note that it is important to see that the conclusion is about a specific time period, regardless of the poetic way of IDENTIFYING that time period. The evidence is that the Pendleton Act "has anchored American government service to competence rather than corruption" for a hundred years starting in 1883.
What are the assumptions? There are at least two. One is that there is no BETTER way to "anchor government service to competence", and another is that the Pendleton Act, which worked well from 1883 to 1983, continued to work well thereafter. In order to evaluate the argument, we need information which could support or refute either of the assumptions. Choice C would do this for the second assumption.
I chose A at first because I thought it helped assess whether a better way exists. Of course, it does not. Choice A would tell us whether other countries have DIFFERENT ways, but not whether they are better. If A had been worded something like "the levels of corruption in the Swiss and British government service since 1883", it would have addressed the first assumption. Even if we did not know HOW they did it, knowing that the levels of corruption in these countries were higher than in the US would make the assumption more likely, while knowing that they were lower would disprove it.
The one aspect of this question which is not well thought out (it is MGMAT, not real GMAC) is the verb "has anchored". This is the perfect tense, which usually means an action in the past WHICH CONTINUES UP TO THE PRESENT. If we read "has anchored" with this normal meaning, it means that the Pendleton Act is STILL doing its job, conflicting with "a hundred years". What then happens is that we interpret "a hundred years" as a piece of rhetoric meaning "a long time", and NOT meaning "until 1983".
If we interpret the evidence that way, C is no longer a correct answer, because it actually conflicts with the evidence as interpreted. In order to avoid this interpretation problem, the verb should have been "anchored" instead of "has anchored".
Kaplan Canada LSAT/GMAT/GRE teacher and tutor