Which one of the following compounds is insoluble in water

Answer:

B

Zinc sulfide is not soluble in water. Its solubility in water is so low that it is classified as water-insoluble. The threshold of solubility is usually at 0.01 g/L, which is slightly more than 0.0001 mol/L for ZnS. According to this document, many values of ZnS solubility in water have been determined and reported, and none of these values reached 0.0001 mol/L.

Dissolving ionic salts in water can be considered as two-step process; destruction of lattice structure, and solvation of water. For ZnS, these two steps can be written like this.

ZnS (s) -> Zn 2+ (g) + S 2- (g)

Zn 2+ (g) + S 2- (g) -> Zn 2+ (aq) + S 2-

The first equation is the reversed process of lattice formation. During the formation of lattice, energy is released, called as lattice energy. This energy depends on many factors. One of these is charge of each ion, which this energy vary directly on these two charge values. This means that lattice energy of salt between 2+ and 2- ion might  be more than that of salt between 1+ and 1- ion, given that other factors are only slightly different. Back to the first equation. The same amount of energy is required to put in to destroy lattice structure. This energy is the reverse of lattice energy. I think there is no specific name for this, though. It is not needed to. In the second process, energy is released. The second process is called hydration, so the name of energy is also hydration energy.
If the amount of energy taken in the first step is bigger than that released in the second, the overall process can be considered as energy-taking, technically called endothermic. Oppositely, if the amount of released energy is bigger, the overall process can be considered as energy-releasing, technically called exothermic. If dissolution process of some salts (like ZnS) is very endothermic, it might be very hard to take place.
in fact, endothermic- or exothermic-ness of reaction cannot exactly determine whether reaction is hard or easy to take place. There are even more factors to consider, but I think I need to stop.