This sounds weird in English. If it's the right one and it is a choice then we can assume there is only one right choice. Therefore we say: that is the right choice NOT that is a right choice.
Can you give me an example of when to would have more than one correct choice?
Even in multiple choice tests there are only ever 1 correct answer, you may need to select many options but unless everything is selected it is not the correct choice.
If this phrase is indeed meant to be used in such a crippling way as you insist then DL needs to make it clear.