You need a field which meets the following requirements:
✑ It indicates the importance of an issue.
✑ It must be used in all projects.
✑ It must allow only a single value.
✑ It must have a default.
✑ It must have different options in different projects.
✑ It must allow options to be disabled.
Which field meets these requirements?
A Select List (single choice) custom field meets all the requirements specified in the question. It allows only a single value to be selected, can be configured to be used in all projects, and can have a default value. Additionally, it can have different options for different projects and allows options to be disabled, which fits the criteria perfectly.
I think the correct answer is B.