Which minor key features 1 flat?

D minor is the correct answer, as it is the only minor key among the options that features one flat in its key signature. In music theory, the key of D minor corresponds to B♭ being present in the key signature.

A minor is an example of a key with no flats or sharps, while G minor features two flats (B♭ and E♭), and C minor has three flats (B♭, E♭, and A♭). Therefore, D minor distinctly stands out with its singular flat, making it the only option that aligns with the question's requirements.