This suggests that even the nutritionists' recommended five servings might not be enough if you're not eating a variety. If anything, this makes the case for vitamin pills stronger, not weaker, because it means the gap between what people eat and what they need might be even bigger than the author thinks.

The fact that some fruits and vegetables contain more nutrients than others doesn't tell us whether most people are getting enough vitamins specifically. Notice that (B) says "nutrients," not "vitamins." Nutrients is a broader category. Even if we set that aside, knowing that some fruits and vegetables are more nutrient-rich than others doesn't tell us whether the people who eat fewer than five servings are eating the nutrient-rich ones or the nutrient-poor ones. Compare this to (D), which provides a concrete alternative source of vitamins ("commonly consumed" in (D) vs. "commonly available" in (B)) that could fill the gap.

Even if nutritionists disagree about what counts as a "serving," the argument's core reasoning still holds: nutritionists say five servings is enough, and most people eat far less than five servings. A disagreement about serving size doesn't change the fact that most people are well below the recommended amount. If most people eat, say, one or two servings, it doesn't matter much whether a "serving" is slightly bigger or smaller than we thought.

This provides an alternative source of vitamins that the author ignores. The author assumes that because most people don't eat enough fruits and vegetables, they must not be getting enough vitamins. But if many common foods that aren't fruits or vegetables are fortified with the same vitamins, then people could be meeting their vitamin needs through those foods instead. The five-servings-of-fruits-and-vegetables benchmark isn't the only path to adequate vitamin intake, so falling short of it doesn't mean you need vitamin pills.

This points out a benefit of fruits and vegetables that vitamin pills can't replicate (fiber). But that actually supports the idea that people should eat more fruits and vegetables. It doesn't address whether most people need vitamin pills to meet their vitamin requirements, which is the only thing the conclusion is about.