Which of the following scale degrees is considered stable in a major triad context?

In a major triad context, the third scale degree is considered stable because it defines the quality of the triad—being either major or minor. The major triad consists of the root, the major third, and the perfect fifth. The major third, being the scale degree that lies a major third above the root, creates a sense of consonance and resolution, which is why it plays a pivotal role in establishing the triad's stable character.

The other scale degrees mentioned don't share this stability in the same way. The second and fourth degrees can create a sense of tension or suspension when involved in harmonic progressions. These degrees often seek resolution to more stable tones, such as the root or third. The seventh degree typically creates dissonance, as it tends to resolve upward to the tonic in dominant seventh chords, further illustrating its instability. Therefore, in the context of evaluating stability within a major triad, the major third as the third scale degree is essential in providing that stable quality.