Correct Answer: cacab

Solution : Given:
_ab _ b _ bc _ ca _

To fill the series we have to divide the series – _ab / _ b _ / bc _ / ca _
Let's check each option –
First option: accbb ; a ab / c b c / bc b / ca b (No repeated pattern has been found.)
Second option: abcca ; a ab / b b c / bc c / ca a (No repeated pattern has been found.)
Third option: cacab ; c ab / a b c / bc a / ca b (In each part the letters are being shuffled – the first letter is moved to the third position in the next part and the second and third letters are moved to the first and second positions respectively.)
Fourth option: abacb ; a ab / b b a / bc c / ca b (No repeated pattern has been found.)

So, the series becomes→ c ababcbcacab. Hence, the third option is correct.