Explanation

Dobereiner's law requires the atomic mass of the middle element to be approximately the mean of the other two. For N, P, As: (14.01+74.92)/2 = 44.47, far from phosphorus's 30.97, so it is not a Dobereiner triad. The other three satisfy the rule: Ca-Sr-Ba (mean 88.7 vs Sr 87.6), Cl-Br-I (mean 81.2 vs Br 79.9) and Be-Mg-Ca (mean 24.55 vs Mg 24.3). Hence N, P, As was not given by Dobereiner.