A polar covalent bond is a bond between two atoms that has a low electronegativity difference (usually 0.1 < x < 1.9, which is kind of the border between nonpolar covalent and ionic, respectively). The most prominent example of a polar covalent bond is water, where the oxygen atom has an electronegativity of 3.5 and the hydrogen atoms have electronegativities of 2.1. The electronegativity difference is 3.5-2.1 = 1.4, which is still in covalent territory, but still a polar bond. This is why lowercase delta's appear beside oxygen atoms in drawings.
Examining, the four options, we immediately see that option D is Cl-Cl. A bond between two of the same atoms inevitably implies an electronegativity difference of zero, which means a nonpolar covalent bond: electrons are equally shared. Anywhere you see two identical atoms bonding (except for metals), you will have a nonpolar covalent bond.
