not A
Every call to runAs counts against the total number of DML statements issued in the process.

not A
Every call to runAs counts against the total number of DML statements issued in the process.

B. runAs() can be used inside of test classes to validate record-level security.
The runAs() method allows you to run test methods as a specific user, which is useful for validating that your application’s record-level security settings (such as sharing rules and object permissions) are correctly enforced. You can simulate different users accessing records to ensure that only the appropriate users can access or modify those records according to your security model.

C. runAs() can be used inside of test classes to validate field-level permissions.
While runAs() primarily affects the user context for the purpose of record access, field-level security must be manually checked within the test by attempting to read or write to fields that the running user should or should not have access to, and then asserting the expected outcomes. This means your tests can include logic to ensure that users have the correct field-level permissions.

E. runAs() does not enforce user permissions or field-level permissions in test classes.
When using runAs(), it's important to note that while it changes the context of the user under which the test is run, it does not by itself enforce user permissions or field-level security. Apex tests run in system context, meaning they have access to all objects and fields—permissions are not enforced unless explicitly coded in the test logic (e.g., using with sharing keyword on classes or manually checking access before performing operations). This is crucial for testing to ensure that your application respects Salesforce's security model when it's expected to....