Which of the following pairs are main group elements?

The question asks about the common characteristic of elements in the same group in the periodic table.

• A. The same number of electron shells: Elements in the same period (horizontal row) have the same number of electron shells. Elements in the same group (vertical column) have an increasing number of electron shells as you move down the group. Thus, this option is incorrect.
• B. The same mass number: Mass number is the sum of protons and neutrons. Elements in the same group are different elements, each with a unique number of protons and neutrons, leading to different mass numbers. Thus, this option is incorrect.
• C. Similar chemical properties: Elements in the same group have the same number of valence electrons. This identical valence electron configuration dictates their chemical reactivity and leads to similar chemical properties. Thus, this option is correct.
• D. The same atomic number: Atomic number is the number of protons, which uniquely identifies an element. Each element in a group has a different atomic number. Thus, this option is incorrect.

Based on the evaluation, elements in the same group exhibit similar chemical properties due to having the same number of valence electrons.

The final answer is $\boxed{\text{C}}$