A major triad is formed by stacking

A. a major third on top of a minor third.

B. a major third on top of a major third.

C. a minor third on top of a minor third.

D. a minor third on top of a major third.

The correct answer is A. a major third on top of a minor third.

A Major triads contain a Major third with a minor third stacked above it, for example, in the major triad C-E-G (which is a C major), the interval C-E is the major third and E-G is a minor third.