Guidewire uses a specific file naming convention to separate base product definitions from customer extensions. For Typelists (which are essentially enums stored in the database), the base definition is stored in a .tti (Typelist Interface) file.
According to Cloud Delivery Standards, you never modify the base .tti file (Option D). Instead, to add new codes to an existing typelist, you create a Typelist Extension file with the .ttx suffix (Option A). The file name must exactly match the base typelist name. Options B and C are incorrect because the _Ext suffix is required for new entities or typelists, but for extending an existing Guidewire typelist, the .ttx suffix is the standard mechanism that ensures the new codes are merged correctly with the original ones during a platform upgrade.