Many consumers think that fresh vegetables are more nutritious than

When "that" is used as a modifier, it takes the same form whether it's describing a singular or a plural noun. For example, "Carrots that are contaminated with E.coli bacteria are best avoided altogether." Here "that" is a modifier, and it describes "carrots," which is obviously plural. That's fine.

It's when "that" or "those" is used as a pronoun that you need to make a distinction between singular and plural. "The carrots in your salad seem to be far cleaner than those in my salad." Now "those" is a pronoun" standing in for the plural "carrots." If we need to refer to a singular "carrot", we would use "that." (More on "that" in this article.)

And as an aside, you are gently encouraged to avoid spending time on SC questions that have multiple underlined portions. This is not something that happens on GMAT questions, and this particular mess is clearly not from an official source. So please don't waste your time on this question at all.

I hope that helps!