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 of 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

A-Wrong- because the phrase "when placed....." does not tell what's placed in a stimulating environment
B-Wrong- because "mice" and "canaries" are not examples of neurogenesis - the growth in their brains & neorons is.
C-Wrong - because "mice's brains" and "canaries' neurons" are not examples of neurogenesis.
D-Wrong - because "the brain growth of mice when placed ..." does not tell us what's placed in a stimulating environment
E-Correct. "brain growth" and "increase in neurons" are the correct examples of neurogenesis.

Answer A: The relative pronoun 'when' should refer to a time; here it wrongly refers back to mice (the noun it is next to); mice should be replaced by the relative pronoun 'that' or 'which' (the latter in independent clauses, the former in dependent ones[as is the case here]).
Answers B and C : As already stated in the thread- the mice and canaries or their brains are not the examples; you are looking for a process here.
Answer D: Same as A; the wrong use also creates ambiguity; 'they' at the end can be seen as ambiguous, relating back to more than one plural noun. The 'when they' is also wordy/redundant.
Answer E: Correct use of the relative pronoun that to clearly refer back to mice and both examples are proper examples showing the process of neurogenesis.