Which is most likely a covalent compound?
lif
mgs
nh3
cacl2

Covalent is when 2 non metals combine. N and H are both non metal so the answer is NH3

NH3  forms a  covalent compound

 explanation

a  covalent compound is a compound formed when two  or more  atoms combine by sharing electrons  pairs. Mostly covalent bond is formed   between two non metals. N (non metal )      combine  with H  ( non metal )   by sharing electrons  pairs. The  NH3 bond  is a   three single  covalent bond since  one valence of each  3 atoms  of hydrogen  is shared  with three electrons  of nitrogen.

They are all decomposition reactions.