b_ac_cc_cb_ab_ac

By Trial and Error method,

Option A

bbac | bcca | cbaa | bcac

The above letters do not form a series. Hence option A is incorrect.

Option B

bcac | bcca | cbba | baac

The above letters do not form a series. Hence option B is incorrect.

Option C

baac | bccb | cbba | bcac

The above letters do not form a series. Hence option C is incorrect.

Option D

baac | accb | cbba | baac

The above letters form a series.

Hence, the correct answer is Option D