There are two main reasons I see why C is incorrect. The first is where it says, “most dietary supplements,†because we don’t actually know then if Activite is one of those.
Secondly, it doesn’t actually hurt our conclusion that Activite is effective. It could still be effective even if I takes longer to become fully effective. This statement doesn’t really impact the plausibility of our conclusion either way.
D, however, opens the opportunity that maybe the product isn't effective because it gives us another reason that maybe it is in the best interest of the company to make this offer.