Answer is: D) Be, Mg, Sr.
Beryllium, magnesium and strontium are alkaline earth metals (group 2 of Periodic table of elements).
The elements have very similar properties.
Alkaline earth metals have in common an outer s- electron shell (two electrons). For example, magnesium atom.
Atomic number of magnesium (Mg) is 12, it means that it has 12 protons and 12 electrons.
Electron configuration of magnesium atom: ₁₂Mg 1s² 2s² 2p⁶ 3s².
