Final answer:
Each molecule listed - NH₃, H₂O, and HCN - is polar because they all have uneven distributions of electrons due to differences in electronegativity between atoms and/or their molecular geometry. Therefore, all the compounds listed are polar molecules.
Step-by-step explanation:
The student is asking about the polarity of specific molecules. A polar molecule has a net dipole moment due to the uneven distribution of electrons caused by atoms with different electronegativities and/or by the molecule's geometry. From the list provided:
NH₃ (Ammonia) is polar because it has a trigonal pyramidal shape with a lone pair of electrons at the top, which causes an uneven distribution of charge.
H₂O (Water) is polar because of its bent geometrical shape and the oxygen atom having a higher electronegativity than hydrogen atoms, creating a dipole.
HCN (Hydrogen cyanide) is polar because the linear molecule has a significant difference in electronegativity between carbon and nitrogen.
Therefore, option D, 'all of the compounds,' is correct as they all exhibit polar characteristics.