Step 1: Understand the word "authentic".
"Authentic" means real, genuine, or true.
Step 2: Evaluate the options.
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(1) GENUINE: Synonym, not the opposite.
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(2) RELIABLE: Implies trustworthiness, not opposite.
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(3) VALID: Also refers to truth or correctness.
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(4) FICTITIOUS: Means fake or imaginary, hence the correct antonym.