The geese that gather at the pond of a large corporation create a haza

The exact wording of the passage and answer choices are key to understanding why (A) correct and (C) is not.

First, note that the plan is to import a large number of "herding dogs" -- this implies that the plan is not necessarily to terrify the geese, but to herd them away from the helicopter and the rather wimpy corporate executives.

With this understanding, take another look at (A):

If the dogs form an uncontrollable pack, how will they herd the geese away from the helicopter? Sure, they may still frighten the geese -- but they would be just as likely to frighten the geese toward the helicopter as away from it. Or the dogs could just run away altogether, leaving the geese to continue menacing the corporate executives. The plan in the passage relies on the dogs performing a very specific task -- keeping the geese away from the helicopter. If they form an uncontrollable pack, they are unlikely to naturally perform this specific task.

(A) casts doubt on whether the plan will be successful, so keep it for now.

Now look at (C):

Think about the timeline implied by this answer choice: before the imported dogs arrive, the geese are a hazard to corporate executives, even though foxes are preying on the old and sick members of the flock. This tells us that the old and sick geese are NOT the primary problem -- the younger and/or healthier geese are the ones threatening our precious executives!

So, by frightening away the foxes that prey on the old and sick geese, the dogs are not necessarily making the situation worse. Furthermore, if the dogs are successful at keeping the geese away from the helicopter, this would presumably include ALL of the geese -- the old and sick as well as the young and healthy. So, the fact that foxes will no longer prey on the old and sick geese does not have a significant impact on how successful the plan will be.

(C) does not cast doubt on the plan's potential for success, so eliminate (C).

Note that we are not looking for an answer choice that spells absolute doom for the plan in question -- we just need to determine which answer choice "cast[s] the most serious doubt on the prospects for success of the corporation’s plan."

(A) casts the most doubt on the plan's potential for success, so (A) is the correct answer.

I hope that helps!