Some airlines allegedly reduce fares on certain routes to a level at

We're looking for the answer choice that "most seriously weakens" the argument in the passage. That means that we're looking for the option that weakens the argument a bit more than the other options. We do not need an answer choice that PROVES that the argument is completely wrong. Something that weakens the argument a bit is enough.

Here's (B):

Would (B) be a better weakener if the words "Airline executives generally believe" were taken out? Sure! But as written, (B) still does weaken the argument. It gives us a reason why competitors would NOT choose to come back. If airline executives believe that coming back on a route they've been driven off will result in the leading airline once again reducing its fares, then we'll have less reason to believe that competitors will return at all.

(B) most seriously weakens the argument because it explains one way that competitors are deterred from disrupting the method in question.

I hope this helps!