The correct option is
C was
Option A: 'Has been' is in the present perfect tense. It indicates an action that began in the past and has just completed in the present.
The Greek philosopher belongs to history now. Hence he cannot complete the action till present 'to be a Greek philosopher'. So, the underlined part has an error and option A is incorrect.
Option B: 'Was' is in simple past tense. It explains an action that happened in the past. It agrees to the given sentence that states history: 'Aristotle' was a Greek philosopher.
Hence option B is correct.
Option C: 'Had been' is in past perfect tense. It states an action that completed in the past, before another action took place. Here, there is no other action that took place. Hence this tense is not suitable.
Option D: 'Have been' is in present tense, where 'have' is the plural form of 'has'. The given sentence does not indicate a present event.
Hence options C and D are incorrect.