To back up all data and attachments in a Salesforce org, the standard approach is to use the Data Export Service.
Option B: Define a Data Export scheduled
Correct Answer.
The Data Export Service allows administrators to schedule a weekly or monthly export of all data and attachments.
The exported data can be downloaded as a set of CSV files and attachments in zip format.
Reference:
Options Not Applicable:
Option A: Schedule a report.
Reports cannot export attachments and are limited to the data accessible via reporting.
Option C: Use the Data Loader command line.
While possible, it would require custom scripting and management.
Data Loader does not natively export attachments.
Option D: Create a Schedulable Apex class.
Writing custom Apex to export data and attachments is complex and unnecessary when standard tools exist.
Conclusion:
The developer should define a Data Export schedule to meet the backup requirement, which is Option B.