要说下为什么,()the book,he found out some answers to this question

选D,此处意为“一~就”,on/upon为介词,后接名词或动词的ing形式.
On/upon reading reading the book,he found out some answers to this question.
若用as,as用作连词,连接一个时间状语从句,意为“当~时候”.应该为 As he was reading the book,he found out some answers to this question.故C不对.
at没有这样的用法.