The type PracticeProblem will have the parent type Quiz. It will take these new property of problemType:

PracticeProblem; Subtype of Quiz: An interactive problem.
problemType; Property of PracticeProblem: The format of practice problem being given. Example: "Multiple choice", "Open ended", "Flashcard". Takes Text as value.

EDIT: per community feedback below, this functionality will be covered by adding eduQuestionType (previously problemType) to the existing Question type. Usage can further be specified by using the existing property of learningResourceType under Question, which will take the Text value of "Practice problem".