Hi,
Do you mean that you would like to change the field "Subject" data type from single line of text to Option Set?

As I know, the field "Subject" is a build-in field and we could not edit its data type. I suggest that you could create a new custom filed with the type of Option Set, then add it to the Task form.
BR,
Judy
---------------------------
Appreciate to Mark as Answer if it is helpful.