Which of the following compounds is known to be soluble in water?

Nitrates are generally well-known for their high solubility in water. This solubility is primarily due to the nature of the nitrate ion (NO3-), which can effectively stabilize in an aqueous environment. Nitrate salts tend to dissociate completely in water, contributing to the overall solubility of the compound.

In contrast, compounds containing carbonate, thiocyanate, and cyanide ions do not exhibit the same level of solubility in water. While some carbonates can be soluble, many are not, especially the carbonates of alkaline earth metals. Similarly, thiocyanates and cyanides can have varying solubility, but they do not match the ubiquitous solubility characteristic of nitrates in aqueous solutions. Therefore, nitrates stand out as the most consistently soluble option among the choices provided.