At the meeting they discussed three different---to the study

approaches是名词,有“方法,途径”的意思,approache常与to连用,是固定词组.而其它选项虽也有方法的意思,却不与to连用,或解释不通.（mean to do是“打算做某事”,way to do是“做某事的方式是”）

不懂还可以问我