Here there are many answer that are parallel, how to choose among all of them? A, C didnt look like correct at all, and I didnt like D as well because it talks about "the brain growth" as if it was some concrete growth... But between B and E i didnt know how to choose... I understand that an example cannot include a mice (it´s obvious ;-p) but can it include a brain growth???

Thanks

Tough one. "examples of neurogenesis include" - so the examples should lead off with whatever the actual thing is that indicated neurogenesis. The mice aren't the example - the example is the brain growth. That eliminates B and C. A also breaks parallelism.

And the other thing wrong with A, B, C and D is "when placed in a stimulating environment" - it sounds like they are referring only to the time that the mice are in the stimulating environment - but the brain growth is permanent. It doesn't shrink back down once the mice are no longer in the stimulating environment. Ditto for the increase in neurons in the canaries.

Original question:
Recently documented examples of neurogenesis, the production of new brain cells, include the brain growing in mice when placed in a stimulating environment or neurons increasing in canaries that learn new songs.

(A) the brain growing in mice when placed in a stimulating environment or neurons increasing in canaries that
(B) mice whose brains grow when they are placed in a stimulating environment or canaries whose neurons increase when they
(C) mice's brains that grow when they are placed in a stimulating environment or canaries' neurons that increase when they
(D) the brain growth in mice when placed in a stimulating environment or the increase in canaries' neurons when they
(E) brain growth in mice that are placed in a stimulating environment or an increase in neurons in canaries that

The OA is E.
However, I think that "or" should be "and" right?
examples....include X or Y.
Isn't it weird?
Or it is okay to use "or" after "include"?

IS IT RIGHT TO SAY THAT CHOICE B, C, & D HAVE AN AMBIGOUS "THEY"?

for choices b and c, yes.
for choice d, no, but do note that it violates 'possessive poison', the rule that everyone loves to hate.

Why are "brain growth" and "an increase in neurons" parallel? Thanks

first, they're logically parallel; that's probably obvious.
note that, true to form, the central noun of the first part is "growth" (not "mice", which disqualifies some of the earlier answer choices), and the central noun of the second part is "increase".

i will assume that you're wondering why the first part doesn't look more like the second part, i.e., why it doesn't say something along the lines of "growth in the brains of..."
this is because "brain growth" is much, much less wordy than that sort of alternative. and, unfortunately, there's really no way to write "an increase in neurons" in the form ADJ NOUN (as is done with "brain growth"), so that's about as parallel as you're going to get.

by the way, i really, really hate this question. i don't think "or" has any business being in this sentence; it should clearly be "and", because both of these are recently documented examples of neurogenesis.
i don't understand how they can write "or" with a straight face.
but, as we've said so many times on here, it's their playground, and they make the rules.