The key of D minor
D E F G A B♭ C
1 flat: B♭. Relative major: The key of F major.
The triads of the key
i
Dm | ii°
E° | III
F | iv
Gm | v
Am | VI
B♭ | VII
C |
The seventh chords
i7
Dm7 | iiø7
Em7♭5 | IIImaj7
Fmaj7 | iv7
Gm7 | v7
Am7 | VImaj7
B♭maj7 | VII7
C7 |
In practice the V of a minor key is almost always major, taken from the harmonic minor: A7 (V7), which carries the leading tone C♯.
D natural minor scale
Open it in the app: 6. Harmony, in D