A Salesforce org has more than 50,000 contacts.

When dealing with large datasets and the need to perform complex calculations or data aggregation, batch Apex and scheduling are typically used to efficiently process records in batches and to execute the process during off-peak hours.

Option B is correct because using the @future annotation allows you to run methods in the background, but it is not suitable for operations that need to process more than 50,000 records, which is the governor limit for future methods.

Option C is correct because implementing the Schedulable interface allows you to schedule the class to run at specified times, such as after business hours, which is a requirement for this scenario.

Option A is incorrect because Queueable is typically used for chaining jobs and handling asynchronous processing of individual transactions, not for processing large datasets at scheduled times.

Option D is incorrect because the Database.Batchable interface is what's needed for processing large datasets, not Database.Stateful. The Database.Stateful interface is used to maintain state across batch job transactions.