The authenticity of the document in question is still being debated by historians.

in 在这里是介词，表示"处于某种状态或情况中"。 question 是名词，意思是"疑问、质疑"。 in question 是一个固定短语，意思是"被讨论的、存疑的"，指正在被质疑或讨论的对象。