Answer: D. one of the atoms sharing electrons is much more electronegative
The polarity of a covalent bond refers to the degree of electronegativity existing between atoms bonded covalently. Generally, a covalent bond becomes polarized when a shared pair of electrons is not equally utilized by the elements existing in such a bond. As a result of the varying electronegativity of the different atoms present in the bond, the pair of electrons shared tends to be more drawn towards the element of a higher electronegativity than to an element of lower electronegativity. In other words, the electrons are not shared equally. Although all covalent bonds between atoms are polar, the degree of polarity, however, varies widely.