I go with D. Just as.. so.. is the correct idiom.

I concur; the correct idiomatic expression is "just as X, so Y."
X:Congress is the legislative branch of...
Y:Parliament is the legislative body of...

The correct response is D.

This question tests the knowledge of idioms
The correct idiom is Just as X so Y
Here:
X:Congress is the legislative branch of...
Y:Parliament is the legislative body of...

Only Option D follows this idiom. Hence the correct answer.

"As X, Y " is also correct form, then why B is not the ans. can any1 pls explain?
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(B) As Congress is the legislative branch of the federal government of the United States,

If you want a clause to compare with another clause thing, you can definitely do so with ‘as’, but necessarily with another comparator such as ‘so’ or ‘as’. Without the second one, the comparison will be incomplete. You do require the ‘so’ after the comma.
In addition, since ‘as’ can be used to mean ‘because’, the choice is vulnerable to some misinterpretation that because, Congress is for the US, Parliament for the UK, without any meaningful correlation.

Therefore, B does not reflect true intention.
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Just like Congress is the legislative branch of the federal government of the United States, so Parliament is the legislative body of the United Kingdom of Great Britain and Northern Ireland.

This question boils down to correct idiom and proper placement of modifier to make parallel comparison
First of all congress should be compared with Parliament.
Secondly the correct idiom is Just as X, So Y
With this knowledge, the correct answer can be vetted in less than 10 seconds.

(A) Just like Congress is the legislative branch of the federal government of the United States, so
Wrong:- Wrong idiom.

(B) As Congress is the legislative branch of the federal government of the United States,
WRONG:- wrong idiom.

(C) As Congress is the legislative branch of the federal government of the United States, in the same way
WRONG:- wrong idiom.

(D) Just as Congress is the legislative branch of the federal government of the United States, so
CORRECT:- Correct idiom ; correct parallelism

(E) Just as the Federal government of the United States's legislative branch is Congress
WRONG:- Wrong idiom; Wrong comparison federal government is compared to parliament.



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KAPLAN OFFICIAL EXPLANATION:

D.

Be on the lookout for connectors like “as,”“just as” and “like.” Here you have to decide which one to use. In everyday speech, this distinction isn’t so impor- tant, but on the GMAT it is. If you’re a native speaker of English, you should be able to use your sense of correct English to tell that “just as . . .so” is correct. Otherwise, jot down this and other commonly-used GMAT idioms as you come across them, and learn them.
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I'm sorry - in understand the idiom, but the sentence really doesn't read right.
Shouldn't is say "Just as, so IS" in this case?

"Just as Congress, so Parliament" really feels like it's lacking a word..

In option E, "his complete lack of formal training and late start" are not parallel. How is this correct? Can you please help explain