Computers are becoming faster, more powerful, and more reliable, and

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."