In which group on the periodic table would a non-metallic element belong if atoms of this element tend to gain two electrons to complete their valence shell?

Group 16. With two electrons, this group completes its octet.

Answer is: 3) 16.

Oxygen (O), sulfur (S), selenium (Se), tellurium (Te) and the radioactive element polonium (Po) are in group 16 of Periodic table.

They have same number of valence electrons (six) and tend to gain two electrons to complete their valence shell.

For example, electron configuration of oxygen atom: ₈O 1s² 2s² 2p⁴.  

Oxygen atom has 8 electrons and tend to gain 2 electrons to gain electron configuration as closest noble gas neon (Ne) with 10 electrons.

a:

by the combination of an unlimited number of elements