Which of the following statements are true regarding the deletion of a custom field? (Select 2 options)

Answer options:

A.You cannot delete a custom field that is referenced elsewhere
B.Once you delete a custom field all data will be permanently deleted
C.You have up to 15 days to restore the custom field and all the data stored
D.You can only deactivate a custom field

Correct Answer: A and C 
To delete custom fields, they shouldn’t be referenced elsewhere. Deleted custom fields and their data are stored until your org permanently deletes them or 15 days has elapsed, whatever happens first. Until that time, you can restore the field and its data. There is no option in Salesforce to deactivate a custom field but you can deactivate the value of a picklist field. 
Reference: 

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