Only the account owner can change their personal account details is the correct statement.

This is true because unmanaged users are personal accounts that are not controlled by an administrator and only the account owner has authority to edit their personal profile and personal account settings.

They are counted as billable users is incorrect because whether an account is counted for billing depends on the service subscription and billing rules and not on the managed or unmanaged status of the account.

They may not have accepted the invitation email is incorrect because acceptance of an invitation is a separate state and cannot be assumed for all unmanaged users.

They belong to at least one group is incorrect because group membership is an assignment that may or may not apply to unmanaged users and it is not guaranteed.

They have access to at least one product is incorrect because product access must be granted and an unmanaged account could have no product access until permissions are assigned.

Full AWS Practitioner Certification Question