Nuclear reactors are sometimes built in “geologically quiet” regions,

I understand when people crib about RC and SC. But the truth is that CR is not hard to rule. All you need is some patience and correct guidance.
You need to find the assumption here. An assumption is a missing premise that is necessary for the conclusion to hold - again, it is necessary for the conclusion. Look for the conclusion of the argument.

Conclusion: Of all potential nuclear reactor sites in such a region, those that are least likely to be struck by an earthquake are ones located near a fault that has produced an earthquake within living memory

Go on to the options now:

(A) Geologically quiet regions are the least dangerous regions in which to build nuclear reactors.
The conclusion categorically states: "Of all potential sites in such a region" i.e. it is only considering quiet regions. It is not comparing geologically quiet regions with other regions so this option is out of scope.

(B) For any potential nuclear reactor site, the likelihood of being struck by an earthquake is the primary determinant of site safety.
Again, the conclusion does not talk about 'site safety'. It only discusses the likelihood of an earthquake. So again out of scope.

(C) In a geologically quiet region, every potential nuclear reactor site is near at least one minor fault.
The conclusion says that of all potential sites in a quiet region, the safest (regarding earthquakes) is the one near a minor fault that has recently produced an earthquake. It does make sense in a way since a recent earthquake would have settled the plates and hence it is not very likely that another earthquake will happen anytime soon. But the question is - 'Is it necessary that there will be a minor fault near all the potential sites?' How about a site which has no faults and hence no chance of an earthquake? Hence, when we say, 'Build the plant near a minor fault which has seen an earthquake in the recent past,' we are assuming that there is no site which has no faults. We are assuming that every potential site has at least one minor fault. Answer (C)

(D) Nuclear reactors that are located in geologically quiet regions are built to withstand at least one but not necessarily more than one earthquake of minor to moderate force.
Again, it is out of scope since the conclusion does not talk about how many earthquakes the reactor can stand. It is only trying to find the site where chances of an earthquake are minimum.

(E) Earthquake faults in geologically quiet regions produce earthquakes at least once in 100,000 years.
We don't need to assume this to make the conclusion true. 1 in a 100,000 is just the average. There needn't be 1 in every 100,000 yrs. The conclusion is only trying to find the most stable area to make the reactor. A fault that has experienced an earthquake recently is expected to be more stable that a site with a minor fault which hasn't experienced an earthquake lately.