i received a private message regarding this thread.

first, there are several errors in this problem statement.
for instance,
* there should be "a" between "speak" and "language";
* "effective" should be "effectively";
* "separately" is spelled wrong.
PLEASE double-check your transcription when you write these things. if you are making this many errors in transcribing the problem statement itself, then we can't count on the fidelity of the answer choices, either.

--

in any case,

* "instead of" ends with "of", which is a preposition. therefore, like other prepositions, it should be followed by a NOUN. ("rather than" has no such restriction; "rather than" can connect essentially any two parallel structures.)
e.g.
i bought a car rather than a truck --> this is ok
i bought a car instead of a truck --> this is ok
but...
my car was large rather than small --> this is ok
my car was large instead of small --> this is not ok, because "small" is not a noun.

there's also a slight difference in meaning (although i am 99.9999% sure that the gmat would never test something so subtle): "X instead of Y" implies that Y is some sort of standard, or is/was expected, but X has been substituted for it. on the other hand, "X rather than Y" just implies a choice between X and Y, with no implication as to any preference or standard.
so, i bought a car rather than a truck means that i had a choice between a car and a truck, and i chose a car. on the other hand, i bought a car instead of a truck implies that i was supposed to buy a truck, and/or that someone expected me to buy a truck, but i bought a car instead.

here, it doesn't make any sense that there would be a pre-existing expectation one way or the other, so "rather than" makes more sense than "instead of".
also, "separately" and "as a separate process" are not NOUNS, so "instead of" is not grammatically viable anyway (i.e., it's wrong).

presumably, the rationale for choosing (a) over (e) is simply that, relatively speaking, (a) is more compact / less wordy than (e).
(note that "wordiness" is NEVER something you should judge in individual answer choices; you should only judge it by comparing answer choices.)