Doctor Liu ______ her patients to have more exercise.

recommend 意為「建議、推薦」。recommend somebody to do something 為牛津學習詞典收錄之英式用法，而 recommend that somebody do 與 recommend doing 則更為普遍。在本題選項中，僅有此字能表達醫生對病人的專業建議，語意完全通順。