Salesforce Admin has created many fields, but he wants to set a few custom fields as Read-Only to some profiles. Where can the salesforce admin achieve this?

Answer options:

A.Field Level Security
B.Workflow Rules
C.Process Builder
D.Page Layout

Correct Answer: A and D 
Options A and D are correct. Users can change a field to be read only through Field level security and Page layouts, but some users are able to edit fields which are Read Only as per Field Level Security or based on the Page Layout. This behavior is seen if the Profile associated with the User has `Edit Read Only Fields` selected. That’s why you need to remove the permission of editing read-only fields on their profiles. 
Option B is incorrect because the workflow is a way to automate certain business processes. You can create a rule based on certain criteria that you set. 
Option C is incorrect because process builder is a point-and-click tool that lets you easily automate if/then business processes and see a graphical representation of your process as you build it. 
Reference:

https://help.salesforce.com/articleView?id=000335725&type=1