Final answer:
All the compounds listed, NH₃, H₂O, and HCN, are polar molecules due to their molecular geometries and the differences in electronegativity between the bonded atoms which create regions of partial positive and negative charges.
Step-by-step explanation:
The molecule listed below that is a polar molecule is b. H₂O. Water (H₂O) is a polar molecule because it has a bent structure due to the presence of lone pairs of electrons around the central oxygen atom. This creates areas of partial positive and negative charges, leading to an unequal distribution of electron density and therefore a permanent dipole moment exists. Ammonia (NH₃) is also polar for a similar reason, having a trigonal pyramidal shape and a net dipole due to the difference in electronegativity between nitrogen and hydrogen atoms. Lastly, Hydrogen cyanide (HCN) is polar as well because there is a significant difference in electronegativity between hydrogen, carbon, and nitrogen atoms, resulting in an uneven distribution of electron density along the molecule, giving it a dipole moment. Therefore, the correct answer is d. all of the compounds.