The correct answer is **完璧 (kanpeki)**. This word means 'perfect' or 'flawless'. In the sentence, 「彼の計画は（　　）で、誰にも文句のつけようがなかった」 (His plan was ( ), and no one could find any fault with it), the key phrase is `誰にも文句のつけようがなかった` (no one could find any fault). This explicitly states that the plan was beyond criticism, indicating a state of being without any defects. **完璧** is the only choice that accurately captures this high degree of quality and flawlessness. Common Mistakes / Nuances: ❌ : **完全 (kanzen)** means 'complete', 'whole', or 'total'. It focuses on whether something has all its necessary parts or is in an absolute state. While a plan can be 'complete' (i.e., all steps are included), it doesn't necessarily mean it's 'perfect' in quality. The sentence emphasizes the *flawlessness* of the plan, not just its completeness. For example, you can have a `完全な計画` (a complete plan), but it might still have small issues. `完璧` is a stronger word for describing impeccable quality. ❌ : **万全 (banzen)** means 'perfectly prepared' or 'thorough', referring to preparations or measures taken to ensure nothing goes wrong. It describes the state of *readiness* for an action, not the quality of an object like a plan itself. For example, you would say `万全の準備をする` (to make thorough preparations) or `態勢は万全だ` (our setup is perfectly prepared). It is incorrect to say a plan itself is `万全`. ❌ : **十分 (juubun)** means 'enough' or 'sufficient'. It implies that something meets the necessary requirements or standards, but it doesn't suggest perfection. A 'sufficient' plan is adequate and gets the job done, but it might not be the best possible plan and could have room for improvement. The phrase `誰にも文句のつけようがなかった` (no one could find any fault) implies a quality that is far higher than merely 'sufficient'.

正解は**完璧（かんぺき）**です。この言葉は「欠点や足りないところがまったくないこと」を意味します。問題文「彼の計画は（　　）で、誰にも文句のつけようがなかった」のキーフレーズは「誰にも文句のつけようがなかった」です。これは、その計画に非の打ち所がなく、誰も批判できないほど優れている状態を示しています。この「欠点がない」という質の高さを最も的確に表現できるのは**完璧**だけです。 Common Mistakes / Nuances: ❌ : **完全 (kanzen)** means 'complete', 'whole', or 'total'. It focuses on whether something has all its necessary parts or is in an absolute state. While a plan can be 'complete' (i.e., all steps are included), it doesn't necessarily mean it's 'perfect' in quality. The sentence emphasizes the *flawlessness* of the plan, not just its completeness. For example, you can have a `完全な計画` (a complete plan), but it might still have small issues. `完璧` is a stronger word for describing impeccable quality. ❌ : **万全 (banzen)** means 'perfectly prepared' or 'thorough', referring to preparations or measures taken to ensure nothing goes wrong. It describes the state of *readiness* for an action, not the quality of an object like a plan itself. For example, you would say `万全の準備をする` (to make thorough preparations) or `態勢は万全だ` (our setup is perfectly prepared). It is incorrect to say a plan itself is `万全`. ❌ : **十分 (juubun)** means 'enough' or 'sufficient'. It implies that something meets the necessary requirements or standards, but it doesn't suggest perfection. A 'sufficient' plan is adequate and gets the job done, but it might not be the best possible plan and could have room for improvement. The phrase `誰にも文句のつけようがなかった` (no one could find any fault) implies a quality that is far higher than merely 'sufficient'.