take care
1. 提防,注意,当心[用作命令式]
2. [口语]保重；再见
watch out
PHRASAL VERBIf you tell someone to watch out, you are warning them to be careful, because something unpleasant might happen to them or they might get into difficulties. (提醒别人) 小心
look out
1. 留神；当心；警惕；注意：
Look out!There's car coming.
当心!汽车来了.
2. 朝外看；把头伸到窗外(看)：
He looked out of the window and saw it was raining.
他朝窗外看,发现下雨了.
3. 找出；挑选出；搜寻：
He looked out some detective story books for his friend.
他为朋友挑选了几本侦探小说.
所以选A