Which of the following is Dobereiner triad

In accordance to Dobereiner, the arithmetic mean of the atomic masses of the first and third elements in a trio, is approximately the value of atomic mass of the second element in that triad when the atoms are ordered in increasing order of their atomic masses.

$\mathrm{Li},\mathrm{Na},\mathrm{K}$ satisfies this law therefore, option A is the correct answer.