What do the atoms of elements in the same group have in common?
a. they have the same atomic numbers.

b. they have the same average atomic masses.

c. they have the same number of electron shells.

d. they have the same number of electrons in their outermost shells.

Option (D).

Explanation:

The modern periodic table is used to study the physical and chemical properties of each element. The modern periodic table shows periodicity in the elements.

The study of the element can be done down the group and along the period. As we go down the group in the periodic table, the number of valance shell increases by one unit but the number of electron in the outermost shell remains same.

Thus, the correct answer is option (D).

c should be the answer