A triad is constructed out of the notes: root, third, fifth.
A triad is constructed out of the notes root, third, and fifth.
A triad is constructed out of three notes. The most common type of triad is the major triad, which consists of the root, third, and fifth of a chord. So, the correct answer is "root, third, fifth". To construct a major triad, you start with the root note, then count up four half steps to find the third, and then count up three more half steps to find the fifth.
