• The word 'authentic refers to something 'real or original', and also the opposite would be ‘unreal or fake’.
  Let us analyze the choices given to us during this question:
• Option (A):  'Factual' refers to something that's concerned with what's actually the case. Therefore, option (A) is wrong because it is synonymous, and not antonymous, with the given the word ‘authentic’.
• Option (B): ‘Trustworthy' refers to someone or something that may be relied on as honest or truthful. Therefore, option (B) is wrong because it isn't anonymous to the given the word ‘authentic’.
• Option (C), ‘original', refers to something that's not a replica. Therefore, option (c.) is inaccurate because it is synonymous and not antonymous to the given the word ‘authentic’.
• Option (D), ‘unreal', means something illusory or imaginary. Therefore, option (d.) is correct because it is comparable to ‘authentic’.

Hence, option D is the correct answer.