Solution: If it is a standard picklist field there are 3 ways to make required or mandatory.
Option 1: Making it required from page layout
Option 2: Create a validation rule to fire if the value is empty.
Option 3: Using trigger add error when the picklist value is empty.
https://salesforceglobe4u.blogspot.com/2020/12/how-to-displayshow-error-message-at.html
If the field is a custom picklist field,along with Option 1,Option 2,Option 3,Option 4 also will be there
i.e directly making it required from the field creation itself.
