What happens to the deleted fields in Salesforce after 15 days?

Answer options:

A.Deleted fields will stay in the backend in Salesforce forever
B.Deleted fields will be automatically erased if not undeleted after 15 days
C.Deleted fields can be restored at any time for 365 days
D.Deleted fields will be automatically erased if not undeleted after 5 days

Correct Answer: B 
Deleted fields will be removed from salesforce after 15 days if they are not undeleted. 
Option A is incorrect because deleted fields will not stay in the backend in Salesforce forever 
Option C is incorrect because deleted fields can’t be restored at anytime 
Option D is incorrect because deleted fields will not be erased if not undeleted after 5 days. It will be erased if not undeleted even after 15 days. 
Reference: 

https://help.salesforce.com/articleView?id=fields_managing_deleted_fields.htm&type=5