Every student in our class had an application form in his hand, but no one knew which office __...

Every student in our class had an application form in his hand, but no one knew which office __...

A

根据句意及句子结构可知，选项部分不定式与疑问词一起作为句子的宾语，在这一结构中，常用不定式的主动结构，可以转换为宾语从句：but no one knew which office he would send it to。注意send sth to sb/sp为固定短语，to为介词，不能省略。