In which set do all elements tend to form anions in binary ionic compounds? li, na, k n, o, i c, s, pb k, fe, br?

Atoms or molecule after gaining of electron possesses negative charge and is known as anion.

For the given sets:

The given elements are alkali metals and have tendency to lose electrons easily and form cations.

The given elements are non-metals and are electronegative. So, they gain electrons easily and form anion.

Carbon has tendency to form bond by sharing of electrons, Sulfur has tendency to gain electrons and form anion whereas Lead has tendency to lose electron.

Potassium and Iron has tendency to lose electron and form cation whereas Bromine has tendency to gain electron to form anion.

Hence, from the given sets, all elements of set: have tendency to form anions in binary ionic compounds.

it means there are 3 atoms of oxygen