The speaker's argument is that CEO pay should depend on performance, not on the size of the organisation, because running a small company also takes a large amount of energy and time and there is no evidence that big-company CEOs face more stress. To refute (attack) this argument, an option must give a real reason why the size of the organisation itself, and not just performance, justifies a difference in pay.
Only the "more challenging" option gives a reason tied to the size of the organisation itself, which is exactly what the speaker denies. This is the one statement that weakens the claim that pay should be based only on performance.
Let's summarize:
So the correct option is "Managing a big organisation is more challenging than managing a small one."