Hey Kristina,
Following the same structure I outlined in your other assumption post -
The conclusion is that catering to these advertisers is against their best interest.
Why?
Because to remain an effective advertising vehicle the magazine needs to maintain editorial integrity.
So there's a gap here in the logic - what if the advertisers don't care about the magazine being an enduring effective advertising vehicle? What if they want to get one good run of ads in and then switch to radio ads? We need a way to bridge these ideas.
Answer choice (C) does this by saying the advertisers must also value the enduring effectiveness of the magazine over short term favorable ads. If this is true, it makes sense for the author to conclude that catering to the advertisers is actually against their best interest we now know that their interest is in the mag enduring as an advertising vehicle!
So with assumption questions, in addition to the negation technique, you can also try taking an answer choice and applying it to the stimulus, like we just did here, and seeing if it acts as a supporter for the conclusion by filling in an obvious gap in the logic. Linking another blog about assumption questions here for you to read through too:
https://blog.powerscore.com/lsat/powers ... he-day-10/
Hope that helps!