Choose the correct cluster of letters to complete the given series.

_bac_ac_acb_ _ba

By Trial and Error method,

Option A

cba | caa | cba | cbb | cba

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

Option B

aba | cba | cba | cba | cba

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

Option C

aba | cca | cca | cbb | bba

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

Option D

cba | cba | cba | cba | cba

The above letters form a series.

Hence, the correct answer is Option D