Final answer:
NH₃ (ammonia) is polar, while BCl₃ (boron trichloride) is nonpolar.
Step-by-step explanation:
In order to determine if a molecule is polar, we need to look at its molecular geometry and the individual bond polarities. If a molecule has polar bonds and an asymmetrical molecular shape, then it is considered to be polar. If all the bonds are nonpolar or if the molecule has a symmetrical shape, then it is nonpolar.
In the given options, NH₃ (ammonia) is polar. It has a trigonal pyramidal shape with a central nitrogen atom bonded to three hydrogen atoms. The nitrogen-hydrogen bonds are polar, and the molecule is asymmetrical, making it polar.
BCl₃ (boron trichloride), on the other hand, is nonpolar. It has a trigonal planar shape with a central boron atom bonded to three chlorine atoms. The boron-chlorine bonds are polar, but the molecule is symmetrical, which cancels out the overall polarity.
Therefore, the correct answer is (b) NH₃.