Answer: The correct answer is "c. certain conditions must be met before it can be concluded that autonomy has been respected and mutual benefit has been achieved.".
Explanation: The simple situation in which two parties come together and freely agree to an exchange is ethically legitimate only prima facie because certain conditions must be met before it can be concluded that autonomy has been respected and mutual benefit has been achieved.
This means that only at first this simple situation is ethically legitimate, because for that to be so, other conditions must be met that really determine that the autonomy of the will has been respected and that each one acts seeking their benefit, therefore , that there is mutual benefit.