我来帮你,其实很简单：
答案是where or to which
to which在这里就等于where,它的先行词是the place ,是地方,所以应该选择where,你再把句子还原,就是Last summer I went to the place is HangZhou.这是还原后的简单句,不及物动词go后要加to,其实这类的介词填写要根据具体的句子而言,比如还有：in which ,on which 等之类的．
希望帮到了您!