How are elements in the same group of the periodic table similar to each other chemically?

Question

a
These elements all have the same number of electrons.
b
These elements all have the same number of energy levels with electrons in them.
c
These elements all have the same number of electrons in their inner shell.
d
These elements all have the same number of valence electrons.

Answer 1

d. These elements all have the same number of valence electrons.