Among these element groups, which is least likely to form ions? : 7a 3a 1a 4a

The  element  group  which  is least  likely  to form ions  is 4A  group

 explanation

Element   of   group  4A   has   4  valence  electrons.  for example Carbon which has   [He] 2S2 2P2  electron configuration has 4 valence electrons.

Since 4A elements has 4 valence electrons ,  it means  that  their valence shell  is half  filled which is  relatively stable. For  this reason 4A element readily  form  ions.