Which type of seventh chord is formed from a minor chord?

The seventh chord that is formed from a minor chord is indeed a minor 7th chord. A minor chord consists of the root, minor third, and perfect fifth. To create a minor 7th chord, you add a minor seventh above the root of the minor triad. This results in a chord that retains its minor quality while incorporating the additional note.

For instance, if you take an A minor chord (A, C, E) and add the G (the minor seventh), it forms an A minor 7th chord, which includes the notes A, C, E, and G.

The other types of seventh chords would involve different alterations either to the chord itself or to the seventh degree. A major 7th chord is built from a major triad by adding a major seventh; a dominant 7th chord arises from a major triad with a minor seventh added; and a diminished 7th chord is formed from a diminished triad with a diminished seventh. Therefore, the characteristics of each chord type clarify why the minor 7th chord is the correct answer given its direct relation to the minor triad.