In the key of C major, the D minor chord is the supertonic. It is the second chord in the scale, derived from the second degree (D) of the C major scale. Here are the functions of each chord in the key of C major:
- The tonic is C major (I).
- The supertonic is D minor (ii).
- The mediant is E minor (iii).
- The subdominant is F major (IV).
So, the correct answer is that the D minor chord is the supertonic.