C. Back-up the Shoe Style values in existing records.
D. Change the field type from a multi-select picklist field to a picklist field.

answers : C,D

The correct options are C & D

D : If you want the user to choose only one value rather than multiple , you can change its type in the object Manager to Picklist Data type

C : You need to backup all the data being stored on the Shoe style field because when you will change the data type you loss all the values being stored on that field

There is no checkbox labelled as "Choose only one value" , i test it in org