To which notes do flats need to be added to make the following scale minor

The form in which this scale in currently is known as D Dorian mode. In order to make this mode minor, you need to add flats to only one of these notes. The note you are looking for is B. In order to turn the whole scale into d minor, you need to add the sign ♭ to B, to get its natural minor scale. 
So, the correct answer is B.

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