Correct Answer: andaa

Solution : Given:–
ab_d_aaba_na_badna_badna

To fill the series we have to divide the series→ab_d_a / aba_na / _badna / _badna
Let's check each option –
First option: babda ; ab b d a a / aba b na / d badna / a badna (No repeated pattern has been found.)
Second option: andaa; ab a d n a / aba d na / a badna / a badna (abadna is repeated in the series.)
Third option: badna; ab b d a a / aba d na / n badna / a badna (No repeated pattern has been found.)
Fourth option: dbanb; ab d d b a / aba a na / n badna / b badna (No repeated pattern has been found.)

So, the series becomes→abadnaabadnaabadnaabadna. Hence, the second option is correct.