Drupal distinguishes between interface text and configuration text . The built-in labels and other text that come from Drupal core, modules, and themes are handled by the Interface Translation system, and Drupal can download those translations for added languages. The Drupal User Guide explains that this built-in text is typically translated by downloading available translations rather than translating it yourself.
By contrast, field labels are configuration. Drupal's multilingual documentation says that configuration text includes items such as labels for fields in your content types , and the Configuration Translation module provides the interface to translate that configuration into other languages. It also specifically lists Content fields as translatable configuration elements. That means field labels you created yourself in Spanish are not automatically translated into French just because French is added; they must be translated through configuration translation.
So, when French is added, Drupal can automatically provide translations for built-in field/interface text where translations exist, but custom field configuration labels still require manual translation . Therefore, option C is the correct answer.