Elements belonging to the same group have similar properties becauseThey have a similar electronic configuration of the outermost shellTheir atomic numbers go on increasing as we move down the groupAll of them are metallic elementsNone of the above

The elements belonging to the same group have similar properties primarily because they have a similar electronic configuration of the outermost shell. This is the most significant factor that determines the chemical behavior of an element.

As we move down a group in the periodic table, the number of shells increases, but the number of valence electrons (electrons in the outermost shell) remains the same. This is why elements in the same group have similar chemical properties, as these properties are largely determined by the behavior of the valence electrons.

While it is true that atomic numbers increase as we move down a group, this is not the primary reason for the similarity in properties. The increase in atomic number means an increase in the number of protons and electrons, but it's the arrangement of these electrons (specifically, the valence electrons) that matters most.

Not all elements in a group are metallic. For example, in Group 17 (the Halogens), elements like Fluorine and Chlorine are non-metals. So, the statement "All of them are metallic elements" is not correct.

Therefore, the best answer to your question is: "Elements belonging to the same group have similar properties because they have a similar electronic configuration of the outermost shell."