（前略）天気は とても よかったです。山を （ ２３ ） とき、きれいな 鳥の 声が 聞こえました。 【質問】（ ２３ ）に入れるのに最もよいものを選びなさい。

Translation of sentence: While we were climbing the mountain, we heard beautiful bird voices. - Grammatical point: Action in progress modifying 'とき' (when/while). - Explanation: '登っているとき' means 'while climbing' (during the active climb). Using the present continuous relative clause is the most natural way to express a state in progress when another event occurred. - Dictionary form '登るとき' means 'right before climbing', which doesn't fit hearing birds during the climb.