_ bba _ ba _ baa_

Given : _ b b a _ b a _ b a a _
Solution : 
Group the given sequence into repetitive segments alternatively
Observe that series has alternate similarity on even and odd places.

Fill the blanks with "abb on odd place and aab" on even place to complete the sequence
abb | aab | abb | aab
Thus, the missing letters are a,a,b,b
Thus the correct answer is (D) aabb