Option 1 -> "Fatal" means deadly or disastrous, which wouldn't lead to acquittal.
Option 2 -> "Fallacious" means logically flawed or deceptive, creating irony that despite being flawed, the argument was convincing enough to succeed.
Option 3 -> "Frail" means weak, which doesn't contrast strongly with "convincing."
Option 4 -> "Fair" means just or reasonable, which doesn't create the necessary contrast with "convincing."
Hence, Option 2: fallacious -> The word "but" signals a contrast, and "fallacious" (containing logical flaws) perfectly contrasts with "convincing" (appearing persuasive). This creates an ironic situation where despite the argument being logically flawed, it was persuasive enough to secure the accused's acquittal -> correct