B; Answer A may be tempting, especially since it is true. However, think carefully about what is actually happening here. Cations are added to a solution containing nitrate, a negatively charged anion, and ammonium, a positively charged cation. Two cations will never interact with each other. Thus, saying that ammonium compounds are always soluble does not logically apply because no compound would be formed anyway. Answer B is correct. The hydrogen bonding capability of nitrates IS a logical explanation for their solubility and ammonium, being a cation itself, would not react with other cations. You may question whether or not nitrate can form hydrogen bonds because there is no hydrogen directly attached to an F, O or N atom. This does mean that nitrate cannot be a hydrogen bond donor and therefore could not hydrogen bond with itself. However, it can act as a hydrogen bond acceptor. Any F, O or N with a lone pair can act as a hydrogen bond acceptor.