Which property is common in both alkali and alkaline earth metals?

Both alkali and alkaline earth metals are indeed good conductors of electricity. This property arises from their metallic bonding structure and the presence of delocalized electrons that are free to move throughout the metal lattice. As both groups belong to the s-block in the periodic table, they have a relatively simple electron configuration that allows for effective conduction of electrical current when an electric field is applied.

In contrast, the other options signify properties that are either specific to one group or do not apply universally to both groups of metals. For example, while alkaline earth metals can lose two electrons, alkali metals predominantly lose one electron. The covalent bonding and high melting points characteristic of these metals are also not shared uniformly across both groups, further reinforcing why the ability to conduct electricity stands out as a common property.