How to split a master file into many excel workbook based on a column

Power Query is not the right tool for this as it doesn't produce different Excel files. You will have to do quite a manual work and save all files one by one after applying different filters in PQ.

Right tool for this is Excel itself if you want automated version. 

I have written a VBA macro in which you need to select the column and it will generate those many files corresponding to distinct entries. Hence, if you run macro after selecting Subjects column or just selecting any cell in Subjects column, you will get two files.

Article 48 – Split A Workbook into Multiple Workbooks on the basis of a Column

If you are doing some transformations in PQ, do it and save the output in an Excel. Then run the above macro.