Which triad type is built from root, minor third, and diminished fifth?

• A. Diminished Triad
• B. Major Triad
• C. Augmented Triad
• D. Minor Triad

Answer: (A)

Triads are named by the intervals from the root to the third and to the fifth. A diminished triad features a minor third above the root and a diminished fifth above the root. That exact stack—root, minor third, diminished fifth—defines the diminished triad. The diminished fifth is one semitone lower than a perfect fifth, giving the characteristic tense sound.

If it were a major triad, the fifth would be a perfect fifth; if it were an augmented triad, the fifth would be augmented; if it were a minor triad, the third would be minor but the fifth would remain perfect. The given structure matches the diminished fifth, so it’s the diminished triad.