B, 51 mL of mystery solution
Are you sure it isn’t SO3+H2O = H2SO4 because that would be combination (synthesis) A+ B=AB
Or SO3 + H2SO4 = H2S2O7
Because that would also be synthesis
I would say that only the only density label is cm^3/g
Lithium has 1 valence electron available for bonding. So its A.