The correct answer is C. Counters.
A counter is an alphanumeric name that corresponds to some current counter value in a document. Counters are used to track the number of times a particular event has occurred, such as the number of times a page has been viewed or the number of items in a list. Counters can also be used to track the progress of a task, such as the number of steps that have been completed in a workflow.
A class is a blueprint for creating objects. Classes define the properties and methods that objects of that class will have. Objects are instances of classes. When you create an object, you are creating an instance of a class.
A key is a unique identifier for a record in a database. Keys are used to retrieve records from a database.
None of the mentioned is not the correct answer.