The correct answer is **有望 (yūbō)**. This word means **'promising,' 'hopeful,' or 'having a bright future.'** In the context of a sentence like 「あの若手選手は（　　）だ。今後の活躍が楽しみだ。」("That young player is (promising). I'm looking forward to their future success."), **有望** is the perfect fit. The second part of the sentence, 「今後の活躍が楽しみだ」 (I'm looking forward to their future success), explicitly expresses anticipation for the player's future, which is the core meaning of **有望**. Common Mistakes / Nuances: ❌ : **偉大 (idai)** means **'great' or 'magnificent.'** This word is used to describe someone who has *already* achieved extraordinary things and is widely recognized for their greatness, such as a historical figure (e.g., 偉大な指導者 - a great leader) or a legendary artist. It's too strong and premature to describe a young, up-and-coming player whose potential is yet to be fully realized. It describes an established state of greatness, not future potential. ❌ : **重要 (jūyō)** means **'important' or 'essential.'** This word describes the significance or necessity of something in a particular situation. For example, 「これは重要な会議だ」(This is an important meeting). While the young player might be an *important* member of the team, **重要** focuses on their current role and value, not on their future promise. The sentence is specifically about looking forward to the future, making **有望** a much better choice. ❌ : **貴重 (kichō)** means **'precious' or 'valuable,'** often implying rarity. It's used for things that are highly valued and not easily replaced, such as 'precious time' (貴重な時間) or a 'valuable experience' (貴重な経験). While a promising player is certainly a 'valuable asset' (貴重な存在) to a team, this word doesn't specifically convey the nuance of future potential. **有望** is more precise because it directly describes the *promise* of future success, which is the central idea of the sentence.

正解は**有望（ゆうぼう）**です。この言葉は**「将来に良い見込みがある、将来性がある」**という意味です。「あの若手選手は（　　）だ。今後の活躍が楽しみだ。」という文脈では、**有望**が最適です。文の後半の「今後の活躍が楽しみだ」という部分は、その選手の未来に対する期待を明確に表しており、これは将来の可能性を指す**有望**の核心的な意味と完全に一致します。 Common Mistakes / Nuances: ❌ : **偉大 (idai)** means **'great' or 'magnificent.'** This word is used to describe someone who has *already* achieved extraordinary things and is widely recognized for their greatness, such as a historical figure (e.g., 偉大な指導者 - a great leader) or a legendary artist. It's too strong and premature to describe a young, up-and-coming player whose potential is yet to be fully realized. It describes an established state of greatness, not future potential. ❌ : **重要 (jūyō)** means **'important' or 'essential.'** This word describes the significance or necessity of something in a particular situation. For example, 「これは重要な会議だ」(This is an important meeting). While the young player might be an *important* member of the team, **重要** focuses on their current role and value, not on their future promise. The sentence is specifically about looking forward to the future, making **有望** a much better choice. ❌ : **貴重 (kichō)** means **'precious' or 'valuable,'** often implying rarity. It's used for things that are highly valued and not easily replaced, such as 'precious time' (貴重な時間) or a 'valuable experience' (貴重な経験). While a promising player is certainly a 'valuable asset' (貴重な存在) to a team, this word doesn't specifically convey the nuance of future potential. **有望** is more precise because it directly describes the *promise* of future success, which is the central idea of the sentence.