Only when _______ home______ that he had left his bag in the taxi he had taken.

A.he returned; did he realize

B.did he return; did he realize

C.he returned; he realized

D.did he return; he realized

答案及解析： A

考查倒装。Only + 状语从句，从句不倒装，主句的主谓要倒装，，故选A。