A
句意:
每个人手里都有一份申请书,但是没有人知道把申请书寄到哪个办公室。
根据send邮寄,application form 申请书,可知,意思是没有人知道把申请书寄到哪个办公室,分析句子可知,knew后跟“特殊疑问词+to do”结构作宾语,send sth. to sp.把某物寄到某地,介词to的宾语是which office,故要用which office to sent it to,可以还原为no one knew which office they should send it to,故选A。