In which of the following chemical compounds are the building blocks molecules?
Answer B is correct because nitric acid is the only compound whose building blocks satisfy the IUPAC Gold Book definition of a molecule, a neutral (uncharged) entity consisting of at least 2 or more atoms. The building blocks of all other given compounds are atomic ($\ce{K+}$, $\ce{Na+}$ and $\ce{Ag+}$) or molecular ions ($\ce{NO3-}$), which by their nature do not fulfill the requirement of being electrically neutral. Per definition, they are molecular entities, i.e. distinct particles which can be identified as separate entities. This term is more general and also includes charged species like ions.