During crossing over, Choose...
pairs exchange pieces, so that each sibling has a different combination of
Choose...
.

Option (a) is the correct answer.

Explanation:

It is known that when we move across a periodic table from left to right there there is increase in number of electrons. As a result, there is increase in atomic mass of the atoms.

This arrangement of atoms in increasing atomic mass is known as periodicity.

For example, in the periodic table sodium is placed at top left below lithium in third period and it has atomic mass 23. Whereas silicon is also a third period element which has atomic number 28.

The chemical properties of each element depends on their group number because members of a group has elements with same valence electrons.  

Thus, we can conclude that periodicity is best defined as the fact that elements increase in atomic mass in a regular way.