The correct answer is **限界 (genkai)**. In this context, the sentence describes reaching the absolute limit of one's physical strength. **限界 (genkai)** specifically refers to the ultimate point, boundary, or limit of one's ability, capacity, or endurance. The phrase **体力の限界 (tairyoku no genkai)** is a very common and natural collocation in Japanese, meaning 'the limit of one's physical strength'. Therefore, it fits perfectly into the sentence: 「２時間も走り続けるなんて、もう体力の（限界）だ。」(Running for two hours straight... I'm already at the limit of my physical strength.) Common Mistakes / Nuances: ❌ : **限度 (gendo)** also means 'limit', but it refers to a limit on a quantity, amount, or an acceptable range, often one that is set or defined. It's about a permissible boundary rather than the absolute end of one's innate ability. For example, you would use it for a credit card's spending limit (利用限度額) or the limit of one's patience (我慢の限度). While one could theoretically talk about a 'limit' to physical strength, **限界** is the far more natural and common choice for the absolute end of one's physical or mental capacity. ❌ : **制限 (seigen)** means 'restriction' or 'limitation'. It implies a limit imposed by external rules, regulations, or conditions. For instance, a time limit (時間制限) on an exam or an age restriction (年齢制限) for a movie. The sentence is describing an internal, personal feeling of exhaustion, not a rule or regulation being applied. Therefore, **制限** is contextually inappropriate. ❌ : **限り (kagiri)** means 'limit' or 'extent', but it's most frequently used in specific grammatical patterns, not as a standalone noun in this manner. For example, in patterns like 「〜限り」(as long as ~) or 「私の知る限りでは」(as far as I know). It can also be used in set phrases like 「力の限り」(with all one's might). To say 「体力の限りだ」 to mean 'I'm at my limit' is unnatural. The word for the final breaking point is **限界**.

正解は**「限界（げんかい）」**です。この文脈では、自分の体力の絶対的な限界点に達したことを表現しています。**「限界」**は、能力、キャパシティ、忍耐などの最終的な到達点や境界を指す言葉です。特に**「体力の限界（たいりょくのげんかい）」**という組み合わせは、「体力が尽きるぎりぎりの状態」を表す、非常によく使われる自然な表現です。したがって、「２時間も走り続けるなんて、もう体力の（限界）だ。」という文に最も適しています。 Common Mistakes / Nuances: ❌ : **限度 (gendo)** also means 'limit', but it refers to a limit on a quantity, amount, or an acceptable range, often one that is set or defined. It's about a permissible boundary rather than the absolute end of one's innate ability. For example, you would use it for a credit card's spending limit (利用限度額) or the limit of one's patience (我慢の限度). While one could theoretically talk about a 'limit' to physical strength, **限界** is the far more natural and common choice for the absolute end of one's physical or mental capacity. ❌ : **制限 (seigen)** means 'restriction' or 'limitation'. It implies a limit imposed by external rules, regulations, or conditions. For instance, a time limit (時間制限) on an exam or an age restriction (年齢制限) for a movie. The sentence is describing an internal, personal feeling of exhaustion, not a rule or regulation being applied. Therefore, **制限** is contextually inappropriate. ❌ : **限り (kagiri)** means 'limit' or 'extent', but it's most frequently used in specific grammatical patterns, not as a standalone noun in this manner. For example, in patterns like 「〜限り」(as long as ~) or 「私の知る限りでは」(as far as I know). It can also be used in set phrases like 「力の限り」(with all one's might). To say 「体力の限りだ」 to mean 'I'm at my limit' is unnatural. The word for the final breaking point is **限界**.