Based on not only

Elena Plescan wrote:Hi, can someone explain to me the rationale behind the correct answer.

A GMAT CAT score is based on not only the number of questions answered correctly but also their difficulty level.

A) on not only the number of questions answered correctly but also their difficulty level
B) not only on the number of questions answered correctly but also their difficulty level
C) not only on the number of questions answered correctly but also their level of difficulty
D) not only on the number of questions answered but also on their difficulty level
E) on not only the number of questions answered correctly but also on their difficulty level

In the construction NOT ONLY X BUT ALSO Y, not only and but also are known as SEESAW CONJUNCTIONS.
The reason is that -- like two people on a seesaw -- X and Y must BALANCE EACH OTHER.
In other words, X and Y must be PARALLEL FORMS.

B and C: not only ON...but also THEIR
E: not only THE NUMBER...but also ON
Here, not only...but also does not serve to connect parallel forms.
Eliminate B, C and E.

A: based on not only the number of questions answered correctly but also their difficulty level
D: based not only on the number of questions answered but also on their difficulty level
Both of these answer choices are valid: each connects parallel forms and conveys a sensical meaning.
The primary distinction is that D discusses the number of questions answered, while the original sentence discusses the number of questions answered CORRECTLY.
The GMAT would never ask us to choose between these two options.
While we should strive not to change the intended meaning of the original sentence, a change in meaning will never be the ONLY reason that an answer choice is incorrect.

I would ignore this SC.
What is the source?

For more practice with seesaw conjunctions, try the following official SCs:
https://www.beatthegmat.com/sc-jazz-pian ... 67643.html
https://www.beatthegmat.com/og-12-kushan ... 92326.html