Hi Sherry,
Let me add a to Jacques' excellent explanation above.
Note that the question stem is a Strengthen—PR, not a Justify—PR because of the presence of the word “most” in the question stem, which weakens the force required of the correct answer. In a Strengthen—PR question, the correct answer will provide a premise that, when applied to the specific situation in the stimulus, helps support the conclusion. Since a principle is by definition a broad rule (usually conditional in nature), the presence of the Principle indicator serves to broaden the scope of the question, which requires a more abstract understanding of the underlying relationships in the argument.
The key to answering this question quickly and efficiently is first to simplify the stimulus, and then to formulate a suitable prephrase. The editorial argues that nuclear power plants are dangerous. Why? The government claims that we are safe from nuclear accidents, but wants to limit the nuclear industry's financial liability in case of an accident. Liability, however, is only a threat if someone is injured in a nuclear accident. So, the argument goes, we have reason to fear accidents in nuclear power plants.
As I mentioned in my explanation of your earlier question, this argument is quite weak: just because the government wants to limit liability in case of an accident doesn't mean that there is actually any danger of a nuclear accident. Answer choice (D) fixes that problem by establishing the following conditional relationship:
Act to prevent something
Real danger that it will occur
If we adopt this principle, and given the government's behavior as it is described in the editorial, we can conclude that there is a real and present danger of a nuclear accident. If answer choice (D) is true, the logical gap in the argument is fixed, and the conclusion - strengthened. This is why answer choice (D) is correct.
Hope this helps!