The correct option is A have never been able to
Option A: 'Have never been able to' is in present perfect tense. It expresses an action that began in the past and continued to the present time. It is relevant to the given statement: the person 'I' has never been able to understand 'Martin' till date.
Hence option A is correct.
Option B: 'Will never been able to' indicates a future event 'will'. However, 'been' expresses the perfect tense, suggesting a future completed action.
'Been' is used in passive form, whereas the sentence is in passive form 'I' being the subject performs the verb 'be able to understand'.
Hence this verb form is incorrect.
Option C: 'Am not able to' expresses the simple present tense. But, the first part of the sentence 'can't understand' already means the same thing: 'can't' expresses 'the ability to understand'. So, the same won't be repeated again.
Option D: 'Was not able to' expresses a past event whereas the sentences are in present tense ' I can't understand...'
Hence options B, C and D are incorrect.