Aside the fact that the big boss always prefers rather than over instead of, beyond GMAT too I will rather not like to use expressions such as opposed to and as against in issues of comparison. In my opinion and as per custom, you can use as against or as opposed to when you are opposed to or against something materially or conceptually. The element of dislike or repudiation should be innate in such contexts. For example, when I say that I am opposed to smoking either within office or in the public or when state that I am against public display of affection, I am meaning to say that I just don’t like them. But you can not use the same expressions to say tha admission in business schools this year has been nearly 150 thousand as opposed to or as against 100 thousands last year as if the relevant years are fighting between themselves. In these cases. proper diction requires to say that admission in business schools has been nearly 150 thousand this year in comparison to ( or as compared to or in contrast to blah, blah )100 thousand last year. Have I made my point?
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In metalwork one advantage of adhesive-bonding over spot-welding is that the contact, and hence the bonding, is effected continuously over a broad surface instead of a series of regularly spaced points with no bonding in between.

instead of --used for replacement
I want coffe instead of tea .
rather than -- used to preference.



A)
= ...advantage of X over Y is that the contact.. is effected continously "over a broad surface" (O) instead of
"a series of regulary spaced points..." (S)
O and S are not parallel. also "instead of" is incorrec here.

D) ...advantage of X over Y is that the contact.. is effected continously "over a broad surface" (O) rather than
"at a series of regulary spaced points..." (S)
O and S are ll... and also rather than makes sense here.

correct me if i am wrong

One more reason that I think goes in favour of D is Parallelism.

Sentence:
In metalwork one advantage [strike]of adhesive-bonding over spot-welding[/strike] is that the contact[strike], and hence the bonding,[/strike] is effected continuously over a broad surface instead of a series [strike]of regularly spaced points with no bonding in between[/strike].

I have struck out all the Middle Men.
Now to maintain the Parallelism, the correct structure should be:
Phrase starting with Preposition < Idiom > Phrase starting with preposition.
Option A: Does not maintain the above structure as 'Instead of' is the preposition and hence the phrase does not start with a preposition
Option B: Same problem structure is not maintained because 'As opposed to' entirely is the idiom.
Option C: Same problem structure is not maintained because 'In contrast to' entirely is the idiom.
Option D: CORRECT - The structure is perfectly maintained as 'Rather Than' is the idiom and hence the next Phrase begins with the preposition 'At'

How to decide between instead of and rather than here??
I think the verb is effected makes it necessary to use rather than and not instead of.

Experts pls comment on that.

I think the point is:

The comparison of contact. Whether contact is continuous or at regularly spaced points. The meaning of the sentence is trying to point out contact done continuously is better than done regularly spaced.

For choices A, B, which sounds pretty reasonable at first glance, you reject them for not having at

Only D makes the meaning complete.

The sentence goes:
contact, blah blah blah, is effected continuously over a broad surface instead of a series
of regularly spaced points with no bonding

Compares contact effected continuously to a series of regularly spaced points. <--- This doesnt make any sense whether it is A, B or C.

Not sure if I am putting my point across correctly.

I will say it has meaning error , rather than any idiom error.

Here, bonding is affected over a surface, hence comparison should be using some form of on.
In other words, parallel structure should same form of word.

A. over a surface is not parallel with a series of
B. again same error as A
C. same error.
D. over is parallel with at, hence right.
E. wrong contrast, changes meaning

Instead of, as opposed to , in contrast with , these all idioms, their X n Y should be parallel

"hence the bonding" is an appositive modifier of the contact since the bonding or adhesion is the effect of the contact. "hence" makes this point clear and we should take them as one unit rather than as two different phenomena.
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This question focuses mainly on the difference between "Instead of" vs "Rather than"
Instead of: It only takes noun as an object and can only put nouns in parallel.
Rather than: It can take either a noun or a clause as its object. It can put nouns, pronouns or entire actions in parallel.
Hence D is correct answer choice