If I understand you correctly, that is not possible for showing the Variable Name in the Prompt for User Input action.
If you have a KM Variable named "Prefix__RootName" used in the Prompt, then it will appear as "RootName".
The results are put into the KM Variable "Prefix__RootName".
Using your variable names, you could use "VarTest__A" and only "A" would be shown, but the results still go into the name "VarTest__A"
If you want to only show AND use "A", then why not just name the variable "A"?