In a major key, the minor diatonic triads are a. mediant, dominant, and submediant. b. subdominant, dominant, and tonic. c. supertonic, subdominant, and dominant. d. supertonic, mediant, and supmediant.

The correct answer is option D (supertonic, mediant, and submediant)

In a major key, the supertonic and mediant triads (ii and iii) are minor, and the submediant triad (vi) is also minor. The diatonic triads in the key of C major are typically identified with roman numerals as expressed in the brackets.

The answer is option c