He claims that he was in the pub with his girlfriend on the night in question.

in 在这里是介词，表示"在...期间"或"在...时候"。 question 是名词，意思是"问题、疑问"，这里指"被讨论或质疑的事情"。 in question 是一个固定短语，意思是"被讨论的"或"有疑问的"，用来指代前面提到过的某个特定时间、地点或事物。