Which of the following set is incorrect in the law of Dobereiner's triads? 

(atomic masses are mentioned in the bracket below)

 [Li=7 u, Na=23 u, K=39 u, Ca=40 u, Sr=87.6 u, Ba=13.3 u, P=30.97  u, As=74.92 u, Cl= 35.5 u, Br= 80 u, I= 127 u]

Dobereiner stated in his law of triads that the arithmetic mean of the atomic masses of the first and third elements in a triad would be approximately equal to the atomic mass of the second element in that triad.

In the given options-

The atomic weight of Arsenic(As) is not equal to the mean of the atomic weights of Phosphorus (P) and Antimony(Sb). So we can say that phosphorus, Arsenic, and Antimony (P, As, and Sb)  triad do not follow Dobereiner's law of triads.

The atomic mass of Phosphorus(P), Arsenic(As), and Antimony(Sb) respectively are 30.97, 74.92, and, and 121.76.

Here, the arithmetic mean of P and Sb=

$$\begin{gathered} \frac{\mathrm{P}+\mathrm{Sb}}{2}=\frac{30.97+121.76}{2} \\ \frac{\mathrm{P}+\mathrm{Sb}}{2}=76.36 \end{gathered}$$

The atomic mass of Arsenic(As) is 74.92 is not equal to the mean of Phosphorus(P) and Antimony(Sb) 76.36. So, it does not follow Dobereiner's law.

In the other options- The atomic masses of the first and third elements in a triad would be approximately equal to the atomic mass of the second element in that triad.

Hence, the correct answer is -'Phosphorus, Arsenic, Antimony'.