Which of the following ions is an oxyanion? 1. f- 2.so42- 3.nh4+ 4.scn-

An anion is a negatively charged ion, meaning that the ion gained one or more electrons, thus became negatively charged.

An oxyanion is an anion that contains at least one oxygen atom bounded to other element(s)

Checking the choices given, we will find that the only oxyanion is:
(SO4)2- 

Ch4 is nonpolar and insoluble in water. since all of the hydrogen atoms are evenly spread around the carbon atom even through the c h bond is polar all the poles cancel each other out.