Thanks for the question! First, let’s take a look at what the stimulus is telling us. We’re told that nutritionists think that people can meet their daily vitamin requirements by eating five servings of fruits and vegetables every day. But most people don’t do that, so the author concludes that most people need to take vitamin pills.
Now we’re asked for a statement that most seriously weakens the argument. Let’s take a look at (A) first. (A) tells us that actually, just eating five servings of fruits and vegetables a day is insufficient, there has to be a variety too. But this strengthens the argument. Remember that the conclusion is that most people need to take vitamin pills. If not even five servings of fruits is enough, and people don’t even get five servings of fruits, then it makes it more likely that people do need to take vitamin pills, which strengthens the argument.
Now let’s take a look at (C). (C) tells us that nutritionists can disagree sometimes on how much of a fruit/vegetable constitutes a complete serving. But even if there is some disagreement about this, it doesn’t weaken the argument, since we know that people eat far less than five servings, and it’s unlikely that the disagreement between nutritionists would be that significant.
Both of these answers pale in comparison to (D), which tells us that people can get vitamins from other sources, which does weaken the argument.
Hope this helps. Feel free to ask any further questions that you might have.