The correct option is D had been traveling
The past perfect continuous form of any verb is obtained by adding 'had been' to the base verb + ing. The verb in question is 'to travel'; for which the base/ root form is simply 'travel'. Applying the above, we clearly get 'had been traveling', which is Option A) - the correct answer. B) is simply past continuous tense; C) is grammatically incorrect and lastly, D) is simple past tense.