Archiving and purging old data from Salesforce on a monthly basis is a good strategy to improve the user experience for the Sales Reps, as it will reduce the clutter and improve the search performance. Creating a permission set or setting data access to private are not effective ways to hide old data from Sales Reps, as they will still consume data storage and affect search results. Using Batch Apex to archive old data on a rolling nightly basis is also not a good option, as it will consume API requests and processing time, and may not comply with the data retention policy.

The correct answer is B, use data classification metadata fields available in field definition. Data classification metadata fields are standard fields that allow you to classify the sensitivity level of your data based on your organization's policies. You can use these fields to indicate whether a field contains confidential, restricted, or general data. These fields are available for both standard and custom objects in Salesforce. Using app exchange products, creating a custom picklist field, or building reports would not be as effective or consistent as using data classification metadata fields.