这道题应该接动词的ing形式,hearing.
不过不能说look forward to 后一定接动词的ing形式,要考虑句子成分以及句子的意思.如：
The letter he has been looking forward to arrived yesterday. 在这个句子中he has been lookingforwardto 充当的是定语成分,后面arrived是句子的谓语.即The letter arrived yesterday.
He looked forward to see what was happening 这句话的意思是：他向前看,目的是看一下发生了什么事.to see 是目的状语.