Which of the following is not Dobereiner’s triad ?

Fe, Co, Ni is transition traid

Dobereiner arranged the elements with similar properties into groups. In this group, when the three elements in a triad were written in the order of increasing atomic masses; the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.
Fe, Co, Ni belong to different groups and have similar atomic masses hence do not constitute a Dobereiner triad.