Hint: Dobereiner’s triads are the group of elements with similar properties. They were discovered by Dobereiner. He proposed that we can form groups of three elements in which all the elements share similar physical and chemical properties.
Complete answer: According to Dobereiner’s law of triads, the arithmetic mean of the atomic masses of the first element and third element in the triad will be approximately equal to the atomic mass of the middle element in that triad. Dobereiner’s law of triads also has limitations. The discovery of new elements made this model obsolete. The newly discovered elements could not be fitted into the triads. Only five Dobereiner’s triads were identified. Many elements could not be fitted into any of the triads. The arithmetic mean of the masses of chromium and beryllium is not equal to the atomic mass of magnesium. So, this group is not a Dobereiner’s triad.
Therefore, option C is correct.
Note: Triad is a term used in music. It is a chord which is made up of three tones called chord factors. A chord is the combination of notes derived from a scale combined to bring out harmony of that scale. Triad is the most basic form of chords. This system of diatonic triads is considered to be the basis of the tonal harmony in music. The law of triads failed because all the known elements could not be arranged in the form of triads.