What will come in place of blank space (_) in the following series.
a d b _ a c _ d a _ c d d c b _ d b c _ c b d _

Given: a d b _ a c _ d a _ c d d c b _ d b c _ c b d _

Now, we check each options.

Option (a): b c c b a (Not Follow)

a d b b a c c d a c c d d c b b d b c a c b d _

Option (b): c b b a a a (Follow)

​a d b c a c b d a bc d d c b a d b c a c b d a

Option (c): b b c a d (Not Follow)

​a d b b a c b d a c c d d c b a d b c d c b d _

Option (d): c c b b a (Not Follow) 

​a d b c a c c d a b c d d c b b d b c a c b d _

So, option (b) is follow the pattern.

Thus, correct option is (b).