Custom fields Remove "None" as an option

[Keith Powe]

I created a custom field "Severity" with the following options:

1 - Critical

2 - Major

3 - Average

4 - Minor

I selected a "2 - Major" as the default value. The issue is that "None" is available as an option. How do I remove "None" as an option so that this field is always filled with one of the values above?

[João Junior]

The field is not marked as mandatory so the option "None" is displayed.
If you make it mandatory the option "None" is hidden.