Correct Answer: addddb

Solution : Given:
_ cb _ ca _ bacb _ ca _ bac_ d

To fill the series we have to divide the series→ _ cb _ / ca _ b / acb _ / ca _ b / ac_ d
Let's check each option –
First option: badddb; b cb a / ca d b / acb d / ca d b / ac b d (No repeated pattern has been found.)
Second option: bbbddd; b cb b / ca b b / acb d / ca d b / ac d d (No repeated pattern has been found.)
Third option: addddb; a cb d / ca d b / acb d / ca d b / ac b d (acbd and cadb are repeated alternatively.)
Fourth option: addbbb; a cb d / ca d b / acb b / ca b b / ac b d (No repeated pattern has been found.)

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