Answer
(a) HCl and (c) CHCl$_3$ are polar.
Work Step by Step
(a) is polar because although chlorine and hydrogen share a pair of electrons, the chlorine atom attracts those electrons much more strongly than the hydrogen. Therefore, part of the molecule will have a partial positive charge and part of the molecule will have a partial negative charge. (c) is also polar for the same reason. (b) is nonpolar because each of the hydrogens around the carbon has an equal attraction for electrons, and (d) is nonpolar because each nitrogen has an equal attraction for electrons.
