After I _, I realized that I had forgotten to tell him about the date of the meeting .A. hanged outB. hung upC. hung awayD. hang on

步骤 1：分析句子结构
句子的主句是"I realized that I had forgotten to tell him about the date of the meeting"，从句是"After I _"。从句中的动词需要与主句的时态一致，主句使用了过去完成时，因此从句中的动词需要使用一般过去时。

步骤 2：分析选项
A. hanged out：闲逛，hanged是hang的过去式，但通常用于绞死。
B. hung up：挂断电话，hung是hang的过去式，用于非绞死的含义。
C. hung away：挂到一边，hung是hang的过去式。
D. hang on：抓紧；坚持，hang是动词原形。

步骤 3：选择正确答案
根据句子的语境，"I realized that I had forgotten to tell him about the date of the meeting"，在意识到忘记告诉对方会议日期之前，应该是挂断了电话。因此，正确答案是B. hung up。