Answer: D — $bb$

The given string is:

$x\, b\, x\, a\, x\, a\, x\, x\, a\, x\, a\, x\, b\, a\, b$

There are $15$ positions, so a pattern repeated more than once can have length $5$, repeated $3$ times.

The last five positions are:

$a\, x\, b\, a\, b$

Using the fixed letters in the string, the repeating pattern is:

$a\, b\, b\, a\, b$

Repeating this pattern three times gives:

$a\, b\, b\, a\, b\, a\, b\, b\, a\, b\, a\, b\, b\, a\, b$

Now, the $7$th and $8$th positions are:

$b\, b$

Therefore, $xx = bb$.