Salesforce admin created a custom field on the opportunity based on Sales Ops request. They would like this custom field to be updated when the stage of the opportunity is set to Closed Lost. How could this be done?

Answer options:

A.Validation Rule
B.Assignment Rule
C.Auto-Response Rule
D.Workflow Rule

Correct Answer: D 
Workflows in Salesforce is a fantastic way to automate certain business processes. You can create a rule, and based on certain criteria that you set, Salesforce can do a number of things, like send an email, create a task, or update a field. 
Other options are incorrect because Validation rules, Assignment rules, and Auto-response rules don’t have the options of updating a custom field. 
References: 

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

Correct Answer: D 
Workflows in Salesforce is a fantastic way to automate certain business processes. You can create a rule, and based on certain criteria that you set, Salesforce can do a number of things, like send an email, create a task, or update a field. 
Other options are incorrect because Validation rules, Assignment rules, and Auto-response rules don’t have the options of updating a custom field. 
References: 

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