Which of the following set obeys dobereiner's triads law?

(atomic masses are mentioned in the bracket below)

 [Li=7 u, Na=23 u, K=39 u, Ca=40.1 u, Sr=87.6 u, Ba=13.3 u, Al=27 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 mean value of Calcium, and Barium is equal to the atomic mass of Strontium.

The Atomic masses of Calcium, Strontium, and barium respectively are 40.1, 87.6, and 137.3 u. 

Here, the arithmetic means of Ca and Ba= $(40.1+137.3)/2=88.7$u.

which is almost equal to the mass of strontium which is  87.6 u.

In the other options- The atomic masses of the first and third elements in a triad would not be approximately equal to the atomic mass of the second element in that triad.

Hence, the correct answer is -'Calcium, Strontium, and Barium'.