【正确答案】正确答案：I showed him a letter which／that／不填I received this morning．

【答案解析】解析：译文：我给他看过一封信。那封信是我今天早晨收到的。后一句话中的it指代letter，故可将letter作先行项，关系分句用which／that引导即可，且在关系分句中作received的宾语，故关系词也可省略。