A peculiar feature of the embryonic mammalian circulatory system is that in the area of the heart the cells adhere to one another, beating in unison and adopting specialized orientations exclusive of one another.
(A) beating in unison and adopting
(B) they beat in unison while adopting
(C) beat in unison, and adopt
(D) beating in unison yet adopting
(E) even though they beat in unison and adopt
B - "they" does not correctly refer to "a peculiar feature"
C - need beating and adopting to modify "a peculiar"
E - "even though" is inappropriate here, "they" is not good like the case in B
D & A are good, but D is better and makes better sense beacuse of "peculiar" that modifies "the embryonic mammalian circulatory system".
So my answer is D.