The question asks us to look at the principle in the argument and explain why there is a contrast in the reactions it describes.
Let's look at the principle. We are told that when a major political scandal erupts before an election and voters blame the scandal on all parties equally, almost all of the incumbents seeking reelection, regardless of party, are returned to office. However, when voters blame such a scandal on only one party, incumbents from that party are likely to be defeated by challengers from other parties.
Answer choice (E) explains why we have this contrast in reactions. This answer explains that when voters blame the scandal on all parties equally, that means that the political scandal is less the responsibility of individual incumbents than of the parties to which they belong. However, when the voters blame the scandal on one specific party, whatever party was responsible must be penalized when possible, i.e. incumbents from that party will likely be defeated by challengers from other parties.
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