The speaker's stand is that CEO pay should depend on performance and effort, not on the size of the organisation being run. To strengthen this stand, an option must give a reason for the pay difference that has nothing to do with size itself, or must remove a common justification people give for a size-based gap.
Between the four, only "CEOs of big organisations are very difficult to hire" ties the pay gap to something other than size, namely the scarcity of the right candidate, which supports the speaker's core claim that performance and capability, not size, should decide pay.
Let's summarize:
So the correct option is "CEOs of big organisations are very difficult to hire."