Hello!

For this question, I would suggest reading each option out loud to yourself. I think you will find that each of the incorrect answers is adding or deleting a word or two, making them either run-on sentences or incomplete fragments.

For each answer, here are the missing pieces:

A: Nothing! It's correct!
B: adding the word "and" after the comma doesn't work. The phrase "...,having amassed a wealth of knowledge over the past twenty years about the brain and its development from birth to adulthood,..." is describing more details about the neuroscientists, not giving us another action they are performing.
C: This would be correct if it was re-written "Neuroscientists are amassing..." to show parallel structure to when they later say the doctors "...are drawing solid conclusions..." Taking out that word makes the sentence into a very long fragment.
D: This sentence DOES need the word "and" after the comma to show that these are two complete sentences being connected together by a comma and a conjunction (for, and, nor, but, or, yet, so). Otherwise it is a comma splice, and the two sentences need different punctuation to connect/separate them.
E: This sentence has the exact same problem as D - it's just worded a little differently. It needs the word "and" after the comma too!

I hope that helps! I'm available if you'd like any follow up.