High blood pressure in the U.S. population is significantly more

C correctly places the modifier "is significantly prevalent" in the right place to refer to high blood pressure. Also, high blood pressure in US is compared with Swiss correctly with the "it is" words.
D is disqualified due to wrong modifier placement. "Is significantly prevalent" in this case refers to population which changes the meaning.

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What follows after comparison has to be parallel.

In option D, to maintain parallelism, replace "it" with phrase,
High blood pressure in the U.S. population is significantly more prevalent than [High blood pressure in the U.S. population] is in the Swiss population.

This is nonsensical. Whereas in C,
In the U.S. population, high blood pressure is significantly more prevalent than [high blood pressure] is in the Swiss population.

This comparison is clear and makes sense.

Request experts to evaluate.

Do we really need to take the whole phrase in place of "it"? Can "It" also refer just to high blood pressure

High blood pressure in the U.S. population is significantly more prevalent than the Swiss population, although the Swiss eat more cheese from France and drink more beer from Belgium than Americans eat and drink.

Official Explanation:

A) High blood pressure in the U.S. population is significantly more prevalent than the Swiss population,

This version illogically compares how prevalent high blood pressure is in the U.S population with how prevalent the Swiss population is.
(Incorrect)

B) In the U.S., high blood pressure in the population is significantly more prevalent than in the Swiss,

Because this version opens with "in the U.S.," what is expressed by the clause that follows "in the U.S" is understood to be the case in the U.S. so, this version is subtly illogical, because it conveys that it is the case in the U.S. that high blood pressure in the population is more prevalent than in the Swiss, as if both "the population" and "the Swiss" are in the U.S. and high blood pressure is more prevalent in the former than in the latter.
(Incorrect)

C) In the U.S. population, high blood pressure is significantly more prevalent than it is in the Swiss population,

Because, in this version, "it" refers to "high blood pressure", this version logically compares how prevalent high blood pressure is in the U.S population with how prevalent it is in the Swiss population, as we can see by substituting "high blood pressure" for "it" in the sentence.
In the U.S population, high blood pressure is significantly more prevalent than it is in the Swiss population

In the U.S. population, high blood pressure is significantly more prevalent than high blood pressure is in the Swiss population
(Correct)

D) High blood pressure in the U.S. population is significantly more prevalent than it is in the Swiss population,

Since the logical referent of "it" in this version is "high blood pressure in the U.S. population," this version illogically compares high blood pressure in the U.S. population with high blood pressure in the U.S. population, as we can see by substituting "high blood pressure in the U.S. population" for "it".
High blood pressure in the U.S. population is significantly more prevalent than it is in the Swiss population
High blood pressure in the U.S. population is significantly more prevalent than high blood pressure in the U.S. population is in the Swiss population.
Also, because "it" refers to "high blood pressure in the U.S population," this version suggests the nonsensical idea that high blood pressure in the U.S. population is in the swiss population.
(Incorrect)

E) In the U.S, high blood pressure is significantly more prevalent than Switzerland,

This version is blatantly illogical because it compare high blood pressure with Switzerland.
(Incorrect)

How do we know "it" refers to "high blood pressure in the U.S population" or "high blood pressure".
Also, in C, In the U.S. population, is modifying the whole clause which does not make sense.

Can someone shed some light on this?

Thanks

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