Before we get to the problem in options D and E, let's look at the intended meaning of choice A. Remember we need to first thoroughly understand the original sentence so that we can properly gauge the other answer choices.

Neuroscientists, having amassed a wealth of knowledge over the past twenty years about the brain and its development from birth to adulthood, are now drawing solid conclusions about how the human brain grows and how babies acquire language.

This sentence indicates the following:
"¢ Over the past 20 years, neuroscientists have collected a lot of knowledge about 2 things - brain and its development
"¢ Neuroscientists are now drawing conclusions about 2 things - growth of human brain and acquiring language.
Key things to notice:

Two actions are being discussed:
a. In the past, scientists collected information
b. Now in the present context, scientists are drawing conclusions

Now let's take a look at choice D:

Neuroscientists have amassed a wealth of knowledge over the past twenty years about the brain and its development from birth to adulthood, now drawing solid conclusions about how the human brain grows and how babies acquire language.

Let's focus on the sequencing of tenses in this sentence. For this, I will omit "now" for now. We will bring this back after this bit of discussion.

Neuroscientists have amassed a wealth of knowledge over the past twenty years about the brain and its development from birth to adulthood, drawing solid conclusions about how the human brain grows and how babies acquire language.

Let's read this sentence now and see what meaning it communicates.
"¢ Over the past 20 years, neuroscientists have collected a lot of knowledge about 2 things.
"¢ The verb-ing modifier can either present result of preceding clause or elaborate on the preceding clause. Let's see which meaning is logical here:
o Elaborating - They collected a lot of knowledge in the past by drawing solid conclusions about 2 things.
 This seems illogical since one does not collect information by drawing conclusions.
o Result - They collected a lot of knowledge in the past and this led to them drawing solid conclusions.
 This seems rather logical. After they collected knowledge, they were able to draw solid conclusions.

But notice one very important thing. In this sentence, both the actions appear to have taken place in the past. The information was collected in the past and the conclusions were also drawn in the past.

Now bring back your understanding of choice A. Per choice A, collection took place in the past but the conclusions were being drawn in the present.

So this is the first reason for rejecting choice D - the shift in the meaning of the sentence.

Now let's bring back the word "now". This word interferes with the logical sequence of actions established in choice D. The logical sequence as we determined was that both actions took place in the past, but the presence of "now" forces the second action to take place in present, thereby creating a conflict.

And lastly, as aditya8062 has pointed out, there is a focus shift in choice D.

Note that modifiers typically do not present the main point of the sentence: they only give additional information. In the original sentence, the subject of the main clause is "Neuroscientists" and the verb is "are drawing". So, the intended focus is to say that neuroscientists are now drawing solid conclusions about something. This focus is now shifted to the collection of information.

Concluding Comments:
It's very important to understand the meaning of the original sentence so that you can assess answer choices in the appropriate light. Remember, grammar is a tool to help you communicate ideas. But there is no use of learning about grammar rules if you do not pay attention to what it is that you have to communicate using these tools.

Regards,
Meghna