His logic seemed absurd.

Answer: Option B

Absurd : wildly unreasonable, illogical, or inappropriate.
Fictional : relating to or occurring in fiction; invented for the purposes of fiction.
Rational : based on or in accordance with reason or logic.
Proportional : corresponding in size or amount to something else.
Useless : not fulfilling or not expected to achieve the intended purpose or desired outcome.

Antonym of Absurd is Rational