Correct Answer: ddaacc

Solution : Given:
a _ ca _ c _ dc _ d _ ad _

To fill the series we have to divide the series – a _ c / a _ c / _ dc / _ d _ / ad _
Let's check each option –
First option: ddacdc; a d c / a d c / a dc / c d d / ad c (No repeated pattern has been found.)
Second option: daadca; a d c / a a c / a dc / d d c / ad a (No repeated pattern has been found.)
Third option: dadaac; a d c / a a c / d dc / a d a / ad c (No repeated pattern has been found.)
Fourth option: ddaacc; a d c / a d c / a dc / a d c / ad c (adc is repeated in the series.)

So, the series becomes→adcadcadcadcadc. Hence, the fourth option is correct.