Answer
$NH_2OH (s)$ is the most water soluble.
Work Step by Step
Let's take a look at each molecules. Recall, like dissolves like.
- $C_{10}H_8 (s)$: Due to the lack of $OH$ groups and the fact that, normally, hydrogens connected to carbons can't make hydrogen bonds, this molecule is considered insoluble in water.
- $NH_2OH (s)$: This molecule is very similiar to water, and since it has one $OH$ group for hydrogens interactions and since it is a small molecule (like water), it should be very water soluble.
- $C_6H_6 (l)$: Same reasons as $C_{10}H_8 (s)$.
- $CaCO_3 (s)$: It is a carbonate, so it is very water insoluble. It has little in common with water molecules.
