NNews and WNews broadcast at same time (so one viewer can view only one program)

Previously: NNews Viewers < WNews viewers
Recently, NNews added sports, weather and personal interest stories.

Now: NNews Viewers = WNews Viewers

Conclusion: the recent programming changes persuaded viewers to switch from World News to Nighttime News.

We need an assumption for the conclusion. That is, what is necessary for the conclusion to hold?

(A) Viewers are more interested in sports and weather than in personal interest stories.

No comparison made between 'sports and weather' and 'personal interest stories'. Irrelevant

(B) The programming content of Nighttime News is more closely aligned with the interests of the overall audience than is the content of World News.

No. Even before we start to evaluate whether it can be an assumption, note that it requires NNews content to be MORE closely aligned. At this time, both have the same number of viewers. So there is no way we need to assume that NNews content is more closely aligned. Perhaps it is aligned to the same level, we don't know. So this is not our assumption. We do not need to assume that NNews content is MORE closely aligned.

(C) Some World News viewers liked the new Nighttime News programming better than they liked the World News programming.

Yes, we are concluding that people switched from WNews to NNews because of new programming changes in NNews. So we are assuming that some people liked the new programming of NNews better than the WNews programming. Then it logically leads to these people switching.

(D) There are other possible causes for an increase in the number of viewers of Nighttime News, including a recent ad campaign that aired on many local affiliates.

This doesn't help our conclusion at all. So not an assumption.

(E) The quality of World News will remain constant even if Nighttime News improves.

Not an assumption. We don't need to make any assumptions about WNews's quality. Even if the quality of WNews programmes is better too, people could have still switched to NNews for their new programming.

Answer (C)