What was Johann Dobereiner's contribution to the development of the periodic table?

He arranged the elements according to increasing atomic mass.

He arranged the elements according to increasing atomic number.

He identified triads of elements that had similar properties.

He organized elements into four groups based on their properties.

He discovered what became known as Dobereiner's triads. For example he noticed that the average of the atomic mass of lithium and potassium was close to the atomic mass of sodium and they had similar properties.

The third choice is the correct one.

He identified triads of elements that had similar properties.

Explanation:

He identified a group of there elements that have relatable physical properties such their atomic weight and density. For example in alkali metals such as lithium, sodium and potassium, the atomic weight of sodium is the arithmetic mean of the first and the third element. The same trend was observed with alkaline earth metals (calcium, strontium and barium), halogen (chlorine, bromine and iodine) and chalcogens (sulphur, selenium and tellurium).