An old friend from abroad, (whom) I was expecting to stay with me,telephoned from the airport
whom 是关系代词, 指代an old friend , 引导定语从句, 在从句中作expecting的宾语
(注意: 从句前后是逗号, 不是句号, 这是非限制性定语从句
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选D.who. 因为who是关系代词,指代 an old friend, 同时friend在句中是做主语的,指代主语用who,而不是用whom;中间的who I was expecting to stay with me 是非限制性定语从句,只是作为一个插入成份,不需要考虑太多。