This is not the greatest GMAT question but let's look at the parallelism going on:

The construction is : One benefit of X rather than Y

In this case X is a noun (learning to speak...while learning to...) so Y should also be a noun. This eliminates the first 3 answer choices as these are adverbs AND do not make much sense because the comparison is between the original process (of learning the two skills together) and the adverbs (separately and independently). Here is an example:

He would rather eat ice cream than quickly

Doesn't make much sense, does it?

What are left are D and E. D is wrong because the original sentence refers to one process, not many, so this would constitute a change in meaning too. Now, although the nouns are very different, they are still nouns; not only this, by using the word 'process' this avoids repetition of what was said in the first half of the sentence.

Thanks .Good Explaination