Unable to start the services of SQL server after Downgrading from Enterprise to Standard
Go to SQL Server Configuration Manager, highlight SQL Server Services in the left side, then right-click this instance on the right side and choose Properties. On the Startup Parameters tab, make sure the locations for -d, -e, and -l are updated and correct. Right now it sounds like they are set to the defaults, which won't work if you've moved the system databases to D:


  


Also it's not clear if you followed the proper process to move the system databases in the first place. Please go through these articles and verify that you done this correctly:


Move System Databases
Move all SQL Server system databases at one time
How to move the Master database