An editorial in the Grandburg Daily Herald claims that Grandburg's

This one is Tough. Answer is D

Here is my Analysis:

Conclusion:
An editorial in the Grandburg Daily Herald claims that Grandburg’s voters would generally welcome the defeat of the political party now in control of the Grandburg City Council.
Premise:
The editorial bases its claim on a recent survey that found that 59 percent of Grandburg’s registered voters think that the party will definitely be out of power after next year’s city council elections.

(A) The way voters feel about a political party at a given time can reasonably be considered a reliable indicator of the way they will continue to feel about that party, barring unforeseeable political developments.

"Feeling about the party" is different from "voting for the party". Even if voters continue to feel about the party the same way as they did earlier, it is not for sure that they will vote against the party.

(B) The results of surveys that gauge current voter sentiment toward a given political party can legitimately be used as the basis for making claims about the likely future prospects of that political party.

"surveys that gauge current voter sentiment toward a given political party " and "survey that the party will definitely be out of power after next year" are different again. Why?
Because, Voters may have sentiment towards some party and can also claim that the party will be out of power next year.
Example: John likes to see congress in power. But he has a gut feeling that congress will have to step down next year.

Assume that out of 100, 75 voters had sentiment toward a political party. But 59 guessed that it would be out of power next year. This does not mean that the 59 would vote against the party. This proportion discrepancy is fixed in option D.

(C) An increase in ill-feeling toward a political party that is in power can reasonably be expected to result in a corresponding increase in support for rival political parties.

Again the same mistake as above. "ill feeling towards party" is different from "assuming party to step down"

(D) The proportion of voters who expect a given political possibility to be realized can legitimately be assumed to approximate the proportion of voters who are in favor of that possibility being realized.

What is the political possibility to be realized here? Guessing that the current party will step down.
What is the proportion of such people... 59 out of 100

So if the conclusion has to stay correct, equal proportion of voters should be voting for that possibility. In other words proportion of voters who are in favor of that possibility being realized should also be 59 out of 100. Only then the conclusion holds true.

(E) It can reasonably be assumed that registered voters who respond to a survey regarding the outcome of a future election will exercise their right to vote in that election.

Even if all the Registered voters exercise their right to vote in the election, we are not sure which party they are voting against. Conclusion cannot hold true.

Hope this helps!!