..for #a.# we got a solution of zinc choride in methanol...the zinc salt would have some solubility in this solvent.
For #d.# you could probably form an adduct between formaldehyde and #BF_3#..certainly we can form the etherate of #BF_3#, i.e. #BF_3*OEt_2#...this is actually distillable, and we would call this a Lewis-acid/Lewis-base adduct....