奮闘

彼の ___ ぶりには頭が下がる。

The correct answer is 奮闘 (ふんとう), meaning strenuous effort or a hard struggle. The phrase "彼の奮闘ぶりには頭が下がる" (I deeply respect his strenuous efforts/struggle) is very natural, as "頭が下がる" signifies deep admiration for someone's admirable efforts, especially when facing difficulties. 努力 (どりょく) is a more general term for effort; while "彼の努力ぶり" is possible, 奮闘 conveys a stronger sense of struggling against odds, making it a better fit for deep respect. 格闘 (かくとう) implies a physical fight or a very intense, often confrontational struggle. 悪戦 (あくせん) is primarily used in the compound "悪戦苦闘" (hard struggle with poor results) and doesn't form "悪戦ぶり" naturally, nor does it typically evoke positive admiration alone.

正解は「奮闘（ふんとう）」です。「彼の奮闘ぶりには頭が下がる」は、困難な状況で懸命に努力する姿に深く敬意を表すときに使う自然な表現です。「努力（どりょく）」は一般的な骨折りを指し、より強い感動を伴う「頭が下がる」という文脈には「奮闘」の方が適切です。「格闘（かくとう）」は主に物理的な戦いや激しい争いを意味します。「悪戦（あくせん）」は「悪戦苦闘」のように使い、苦しい戦いを表しますが、単独で「〜ぶり」とは結びつきません。「奮闘」は逆境に立ち向かう姿勢を称えるのに最適です。