"config" folder is used for configuration to be applied if runmode/environment specific configuration are not present. "config.prod" will be specific to prod runmode
Both B and C are not valid as anchor href was used without @extension. Now A and D are almost same just in A !grandchildren is used and in D !child.listchildren isused. A seems more appropriate as its value was already calculated in first data-sly-test
5. B Changes in core components are never directlu done even if it is used as it is. the are always used by extending.