The cause of periodicity of the properties is:

• (A) Increasing atomic radius
• (B) Increasing atomic weight
• (C) Number of electrons in the valency orbit
• (D) The recurrence of similar outer electronic configuration

The periodicity of properties in the periodic table is primarily due to the recurrence of similar outer electronic configurations. This means that elements in the same group have similar chemical and physical properties because they have the same number of electrons in their outermost shell.

Key Factors Influencing Periodicity

• Atomic Radius: While atomic radius does change across periods and groups, it is not the main reason for periodicity.
• Atomic Weight: Atomic weight varies, but it is not directly responsible for the periodic trends observed.
• Valency Electrons: The number of electrons in the valency orbit contributes to the properties, but the similarity in outer configurations is more significant.

In summary, the correct answer is (D) The recurrence of similar outer electronic configuration, as it best explains why elements exhibit periodic trends in their properties.