Which of the following pairs of elements could possibly be in the same group? x has a 1+ ion; y has a 1- ion. x tends to form a 2+ ion; y tends to gain two electrons. x will tend to gain one electron; y has eight valence electrons. x has an atomic number 15; y forms a 3- ion.

Answer = X has Atomic Number 15 and Y has a -3 ion.

First let us discuss all options,
Option 1 is incorrect because X has +1 ion means it belongs to group 1 while Y has -1 ion means it belong to group 7 (Halogens).
In option 2 X is getting oxidized by loosing 2 electron and forming +2 ion hence metallic in nature, while Y is gaining 2 electrons and forming _2 ion.
Option 3 is also incorrect because X has potential to gain one electron, means it has 7 electrons in valence shell, while Y is already having eight electrons.
Last option is correct because X has atomic number 15 (electronic configuration 1s2, 2s2, 2p6, 3s2, 3p3) means it can gain 3 electrons to complete its octet, and Y has gained those three electrons and has -3 charge.