I think the answer should be D.
'yet' sounds acceptable here as the two statements carry some opposite shades.
It says that the
cells beat in unison.
the cells differ from each other.

Hence, to convey both a information in a single sentence 'yet' is a perfect connector.

eg : She is a genius yet hard-working.
Here also both the adjectives carry opposite shades, 'yet' connects them perfectly.

Hope this helps!

I agree D is the best answer because:
(B) and (E) are totally wrong. We don't need 'they' or a clause with 'even though'.
(C) is the simple form of verb, therefore, it can't be used after a comma.
I end up with 2 option (A) and (D). The difference with those two is the linking verb 'and' vs 'yet'.
'Yet' is used when the author would like to emphasize the latter part of the sentence. Therefore, 'yet' fits better than 'and' in term of meaning in this sentence.
How do you think ?