Select the combination of letters that when sequentially placed in the blanks will create a repetitive pattern.

_ b a a _ a a _ a a b _

Given: _ b a a _ a a _ a a b _​

Now, we check each options.

Option (a): a b b a (Follow)

a b a  / a b a / a b a / a b a​

Option (b): a a b b (Not Follow)​

a b a / a a a / a b a / a b b​

Option (c): a b a b (Not Follow)​

a b a / a b a / a a a / a b b​

Option (d): b b a a (Not Follow)​

b b a / a b a / a a a / a b a​

So, the correct pattern follows is: a b a

Thus, correct option is (a).