Lower courts generally accept what the higher courts say is true not o

No, say is not a noun in the original sentence. It's just that it's not necessary for that portion to be what the what is really introducing. Perhaps the what is actually followed by is, and the the higher courts say portion is not important. I found a sentence very similar to mine in the WSJ:

But these reforms have been financed by what some economists say is a dangerous rise in government debt issuance...

This is meant to be read as:

But these reforms have been financed by what some economists say is a dangerous rise in government debt issuance...

OFFICIAL SOLUTION

When you see a "not only...but also" construction (or a "both...and" or "either...or"), you should immediately seize on the fact that the two components of that construction must be parallel. One helpful trick for rooting out choices that are not parallel is to read the sentence without everything from "not only" through "but also" to see whether what remains is a complete, coherent thought. If it is not, then you know that the construction is not parallel. For example, if you did that with choice A you would have:

A) Lower courts generally accept what higher courts say is true...rules of procedure.
Since this is not a coherent sentence, A is incorrect. If you perform that on B (the correct answer), you will have:

B) That lower courts generally accept what the higher courts say is true about rules of procedure.

This is a complete thought (more on why, if you didn't see that at first, in a minute), so you cannot eliminate B for parallelism.
Choices C, D, and E each an error when held to that standard:

C: That lower courts generally accept what the higher courts say is about rules pf procedure.
D: Lower courts generally accept about legal rules of procedure.
E: Lower courts generally accept what the higher courts say rules of procedure.

Regarding correct choice B: the testmaker here uses an interesting sentence structure that may at first sound incorrect. Your ear may very well want to hear that the lower courts accept what the higher courts say is true (about both the law and rules of procedure). But what the sentence actually says in B is lower courts generally accept what the higher courts say, a fact that is true about both the law and rules of procedure. It's a sentence structure akin to "That it is raining is true in both New York and Boston." Note that the GMAT testmaker likes to find sentences that are correct but also interestingly structured, rewarding those who can unpack a strange sentence to determine what it really says.

The correct sentence structure should be--> Not only X but also Y

Option A-- Lower courts generally accept what the higher courts say is true not only about the law but also about
the missing about is what makes option A wrong

Option B-- Satisfies the above rule

A has two working verbs without a connector [ I am not aware of its technical name, some define it as a run on sentence, but I am not sure]
Lower courts generally accept [what the higher courts say] is true not only about the law but also rules of procedure.
With "that" in B, we are using a noun clause acting as a subject, as below...

Its noun clause - That the GMAT is tough cannot be denied.
Here, "That the GMAT is tough" is a noun clause, clause that act as a noun.

Lower courts generally accept what the higher courts say is true not only about the law but also rules of procedure.
the first part is a prepositional phrase, while the second is a noun. Another reason to Eliminate A

Lower courts generally accept what the higher courts say is true not only about the law but also rules of procedure.

A. Lower courts generally accept what the higher courts say is true not only about the law but also
B. That lower courts generally accept what the higher courts say is true not only about the law but also about
C. That lower courts generally accept what the higher courts say is not only true about the law but also about
D. Lower courts generally accept not only what the higher courts say is true about the law but also about
E. Lower courts generally accept what the higher courts say not only is true about the law but also

Parallelism marker: Not only... but also...

Now in parallelism the elements of the list should be same in means of not only comparison but the structure also.

in the sentence A the ' not only about the law but also rules of procedure'. ther should be a 'about' after but also.

Option B corrects that error. and is the correct answer for this question.

What I mean by working verb is a verb carrying tense. Sorry, may be I used the wrong term for it. I should have used the term "finite verb".
Ok, So "say" is a noun here ? Something like, Jim does have his say in XXXX


I very much got this essence, considering "say" as a noun and as a verb, as discussed above.

Awesome discussion !! Thank you very much.

I understand your point - "some economist say" is acting as some kind of modifier.
But, at the same time, I am really confused how can this type of structure work ? Because, we are using a clause without any so called "connector" (conjunction, Subordinators, and so on).
Have you seen any official problems using this same kind of construction?

Thank you very much.

I'm still not clear on the role "that" plays at the beginning of the sentence. Can someone please clarify as to why that's correct?

+1 for B.

That lower courts generally accept what the higher courts say is true not only about the law but also about- the answer choice maintains the parallel structure and IS (in bold) is the verb for subject THAT.

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