The correct answer is **明白 (meihaku)**. In the sentence 「（　　）な証拠が見つかったので、警察は彼を逮捕した。」 (Because ( ) evidence was found, the police arrested him.), we need an adjective that means 'clear', 'obvious', or 'indisputable' to describe the evidence. **明白 (meihaku)** is a na-adjective meaning 'clear,' 'obvious,' or 'undeniable.' It is often used in formal or written contexts to describe facts, evidence, or mistakes that are so clear they leave no room for doubt. Therefore, 「明白な証拠」 (meihaku na shōko - clear/indisputable evidence) perfectly conveys the meaning that the evidence was conclusive, leading to the arrest. Common Mistakes / Nuances: ❌ undefined: **単純 (tanjun)** means 'simple' or 'uncomplicated.' While grammatically possible to say 「単純な証拠」 (simple evidence), it changes the nuance. It would imply that the evidence itself was not complex, rather than that it was 'conclusively clear' or 'irrefutable.' The context of an arrest strongly suggests the need for decisive, undeniable proof, which '明白' captures far better than '単純'. ❌ undefined: **率直 (sotchoku)** means 'frank,' 'candid,' or 'direct.' This adjective is used exclusively to describe a person's personality, speech, or opinions. For example, 「率直な意見」 (sotchoku na iken - a frank opinion). It is never used to describe inanimate objects or concepts like 'evidence' (証拠). Therefore, it is incorrect in this context. ❌ undefined: **素直 (sunao)** means 'obedient,' 'docile,' 'honest,' or 'upfront.' Similar to '率直,' this word describes a person's character, disposition, or feelings. For example, 「素直な子供」 (sunao na kodomo - an obedient child) or 「素直に謝る」 (sunao ni ayamaru - to apologize honestly). It cannot be used to describe the state of evidence, making it completely unsuitable for this sentence.

正解は **明白（めいはく）** です。問題文「（　　）な証拠が見つかったので、警察は彼を逮捕した。」では、証拠が「はっきりしていて、疑う余地がない」ことを示す形容詞が必要です。 **明白（めいはく）** は、「疑いの余地がないほどはっきりしているさま」を意味するナ形容詞です。特に、事実・証拠・誤りなどについて、誰の目にも明らかである、というフォーマルな文脈でよく使われます。したがって、「明白な証拠」は「決定的で疑いようのない証拠」という意味になり、逮捕の理由として文脈に最も適しています。 Common Mistakes / Nuances: ❌ undefined: **単純 (tanjun)** means 'simple' or 'uncomplicated.' While grammatically possible to say 「単純な証拠」 (simple evidence), it changes the nuance. It would imply that the evidence itself was not complex, rather than that it was 'conclusively clear' or 'irrefutable.' The context of an arrest strongly suggests the need for decisive, undeniable proof, which '明白' captures far better than '単純'. ❌ undefined: **率直 (sotchoku)** means 'frank,' 'candid,' or 'direct.' This adjective is used exclusively to describe a person's personality, speech, or opinions. For example, 「率直な意見」 (sotchoku na iken - a frank opinion). It is never used to describe inanimate objects or concepts like 'evidence' (証拠). Therefore, it is incorrect in this context. ❌ undefined: **素直 (sunao)** means 'obedient,' 'docile,' 'honest,' or 'upfront.' Similar to '率直,' this word describes a person's character, disposition, or feelings. For example, 「素直な子供」 (sunao na kodomo - an obedient child) or 「素直に謝る」 (sunao ni ayamaru - to apologize honestly). It cannot be used to describe the state of evidence, making it completely unsuitable for this sentence.