What you're training
A seventh chord is a triad plus one more note stacked a seventh above the root. Same root, same register, and the seventh (plus the triad underneath it) is the whole audible difference: Cmaj7 and C7 share a major triad but differ by a half step in the seventh; Cm7 and Cø7 share a minor third but differ in the fifth. This drill isolates that one variable so you learn to hear the seventh itself, not just "major vs. minor."
Two levels, one skill
Level 1 is a four-way choice, always rooted on C4: Cmaj7, C7, Cm7, or Cø7 (half-diminished). Level 2 adds a fifth option, C°7 (fully diminished) — the tensest of the five, and only a half step away from Cø7, so the ear has to catch that one note.
Go deeper
Minor sevenths outnumber dominant sevenths in real music — the census → breaks down which seventh chords composers actually reach for, and how often. The theory: seventh chords → covers how each one is built and notated. Voice that same chord differently → is the other half of this pair, once you can already name a seventh chord in root position.
Where it fits
Comfortable with chord quality already? Seventh chords are the same triads you already recognize, plus one more note on top — the skill is catching that extra color, not re-learning the chord. The full drill ladder has everything else.