Some of the letters are missing, which are given in the order as one of the options below it. Select the correct option.
$$\text{\_\_\_\_\_\_ \_\_\_\_\_\_ a b a \_\_\_\_\_\_ \_\_\_\_\_\_ b a \_\_\_\_\_\_ ab}$$

Let's test the substitution of choices to identify a uniformly recurring pattern block.
If we substitute option B (a, b, b, a, b) into the five consecutive empty blanks:

* 1st blank $\to$ a
* 2nd blank $\to$ b
* 3rd blank $\to$ b
* 4th blank $\to$ a
* 5th blank $\to$ b

The entire string translates to:

$$\text{a b a b a b a b a b a b}$$

By inspection, the sequence forms a perfectly repeating 2-letter cycle: ab | ab | ab | ab | ab | ab.
Therefore, abbab is the correct solution.