For a moment, let's remove "too" from answers A-C and take another look. I think you'll see that we can eliminate most of the answers quickly:

A and D are guilty of a comma splice (joining two independent clauses with a comma). Out!
E isn't clear when it says "and also modems." We wouldn't say something like "My son is smart, handsome, and accomplished, and also my daughter." Or at least that would mean something very different! Also, "the devices to allow" is not idiomatic.
C has a similar problem at the outset. What does "and so modems" mean?

That leaves us with B. Computers are becoming XYZ, and so are modems.

Okay, so what about the word "too"? Short answer--it doesn't matter! We can say "I want a million dollars, too" or "I too want a million dollars." Similarly, we can say "So are modems, too" or "So too are modems." Looking at that first option, it actually seems a bit redundant, so we might wonder why we'd bother to add "too" here at all. It's definitely not necessary, but "so too" is a relatively common usage in more formal writing or narration. You are very unlikely to hear anyone say that in conversation! Also, I think the GMAT was stretching a bit here, in that you'd be more likely to see it at the beginning of a sentence, when you don't already have "and" connecting the ideas: "The mayor resigned in the face of the scandal. So too did the members of the city council who had supported his scheme."
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Computers are becoming faster, more powerful, and more reliable, and so too are modems, the are the devices to allow two or more computers to share information over regular telephone lines.


- the first part of the sentence is being built by using words like faster... powerful.. etc...
and then when you compare modems to it, ideally you should maintain the same tone. also falls flat.
-the other options have an obvious error, so at times you just have to listen to your ears..

so too preferred over also?

so too are modems, the devices that allow
also are modems, the devices that allow

The word "so" is important here. This one word replaces the whole "becoming faster, more powerful, and more reliable". Without "so" the sentence would be incomplete since it would not be clear what the modems are.

GMAC, normally, doesn't put more than 3 items in a list! If you have more than 3 items in a list you should suspect the list! We already have 3 items (green part of non-underlined part) in the list. So, look for another things other than item of a list.
So, cross out choice c,d, and e from the choices for making the "modems" as item (without using verb).
A is just run-on sentence.
In B, "the devices...." is going to introduce "modems". B is sweet. So, keep it.

Lots to observe in this sentence. The || here is 'Compters are...so too are modems'.
Next, there is an issue of something called comma splices. 'they are the devices...' is an independent clause so one cannot use a comma to separate two independent clauses. Either use a period, a semi-colon, or rewrite the sentence in a way that includes only one independent clause (with dependent clauses perhaps).

Computers are becoming faster, more powerful, and more reliable, and so too are modems, they are the devices to allow two or more computers to share information over regular telephone lines.

(A) so too are modems, they are the devices to allow X
(B) so too are modems, the devices that allow CORRECT
(C) so too (are) modems, the devices allowing X
(D) also modems, they are the devices that allow X
(E) also modems, which are the devices to allow X
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