The correct option is D Elements in a triad show similar properties.
Dobereiner stated that when elements with similar properties are arranged in a group of three (triad) in the increasing order of their atomic mass, the atomic mass of the middle element is roughly the average of the atomic masses of the other two elements. He could form only 5 triads out of the discovered elements.
However, in Newlands' octave every eighth element had properties similar to that of the first element.