what does it mean when you have a service d message

Well... It means your vehicle is due for service "D". Your owner's manual can explain your service system to you, but the basics are someone who knows how to access the service system will need to pull up the particular services your car is calling for, and then if the services are performed, reset the system. It's just maintenance services, they should be done, but no damage will occur if you drive the vehicle while arranging a schedule to perform the services.

MV

MB went back to straight A's and B's a while ago. It's a lot simpler all round. The service sheet will tell you what items need attention at various time/mileage intervals.