_____ about what he did,he stood there,not knowing what to say or to do.

A
作状语表伴随状态,此时一般用过去分词（因为这里带被动含义）
再如：
He left angrily,determined not to return.
Dressed in white,he went out.
Lost in thought,he almost ran into a tree.为什么C不行呢？可以做原因状语啊！！！求解这个从语法上很难说清的。 你多积累些这样的例子就自然明白了！！这就是语言！！