He came back late,---which time all the guests had already l

B
首先,从语义上分析,客人们肯定在john回来之前已经离开,之前是by the time
而且,had left是过去完成时,which引导定语从句,which指代john回来那个时候,提示过去时间同时也是过去完成时的标志词组的也是by the time