This scenario presents two distinct requirements: processing a large volume of data (50,000 records) and running the process at a specific time (once a day after business hours).

Database.Batchable (Option B): Processing 50,000 records in a single synchronous transaction will exceed Apex Governor Limits (specifically the CPU time limit or the heap size limit). Batch Apex allows the system to process these records in smaller chunks (batches), ensuring that the calculation completes without hitting limits.

Schedulable (Option C): To ensure the job runs "once a day after business hours," the Schedulable interface is required. The developer allows the class to be scheduled via the Apex Scheduler or the UI.

The standard pattern for this use case is to create a class that implements Schedulable, and inside its execute method, instantiate and execute the Batch class (Database.executeBatch). This combines precise timing with bulk processing capabilities.