()Looking---at his mother ,the little boy looked---

选D
第一空look at his mother中的look是个实意动词与at一起构成词组look at“看,瞧”,因此要用副词sadly来修饰.
第二空the little boy looked中的look意思是“看起来”是个系动词,后面要跟形容词sad来做表语.