With Sharing is related to the others data. enforce sharing rule.
WITH SECURITY_ENFORCED is an soql statement it will not update anything.
innerclass is not related this context.
IsUpdateable correct because update is an DML statement and before dml we need to check isupdatetable.

A is the correct answer as isUpdateable can be used on fields to check if a user has edit/update access to that field.

WITH SECURITY_ENFORCED is just for soql retrieving records & with sharing keywords have to do with record level access and not field or object level access.

The correct is A

Answer seems to be C.
If the class is updating the fields for a user with field permission of read only, then class is running in System context. In order to enforce user context, "with sharing" should be used in the class.

A is correct. C or D is incorrect as sharing keyword applies to visibility of a record and not for field level security. SECURITY_ENFORCED is only for querying and not for updating.

A is correct, is Updateble method is there at field level as well.. Schema.sObjectType.Contact.fields.Email.isUpdateable()
ref- https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_perms_enforcing.htm

I believe the answer should be either "C" or "D"
- IsUpdateable is object based
- WITH SECURITY_ENFORCED is useful only during queries, not while updating of records