In the series _b_a_ba_b_abab_aab; fill in the six blanks (_) using one of the following given four choices such that the series follows a specific order.

The correct option is ababab.

To find the correct pattern, let us analyze the given letter series with the six blanks:

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

The total number of position slots (letters + blanks) is 18.

Let us substitute the letters from the correct option ababab into the six blanks sequentially:

1st blank: a
2nd blank: b
3rd blank: a
4th blank: b
5th blank: a
6th blank: b

Substituting these into the series gives:

a b b a a b a b b a a b a b b a a b

Now, let us divide the 18-letter series into 3 equal repeating groups of 6 letters each:

Group 1: a b b a a b
Group 2: a b b a a b
Group 3: a b b a a b

We can see that the sequence forms a perfectly repeating pattern of the 6-letter block abbaab.

Therefore, the letters that fill the blanks in order are ababab.