So in this example question it says which of the following compounds would not be associated with the term formula unit. So remember formula unit is just a general term we're going to use in terms of a compound that possesses a metal plus nonmetal. So we just have to look for the choice that does not contain a metal with nonmetal.
If we look at option A, we have Na2CO3, Na is a metallic sodium. Carbon and oxygen are both non metals, so this would represent a compound that could be termed a formula unit. Next we have this long compound here, but again it has sodium in it and all these elements here are nonmetals. So this could also use the term formula unit.
See though, see has B which is boron. Boron is a metalloid and it has F which is fluorine which is a nonmetal. So this would be our answer. We could not call. We cannot associate the term formula unit with this compound because it contains a metalloid and not a metal. And then finally we have Mn and Cl. Mn is manganese which is a transition metal and Cl is chlorine which is a nonmetal. So this could be associated with the term formula unit.
Out of all the choices, only option C couldn't be associated with the term formula unit.