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In a water molecule, there is one oxygen atom bonded to two hydrogen atoms. The molecular formula of water is H2O. The oxygen atom has six valence (outer shell) electrons, whereas hydrogen atoms have one valence electron each. The oxygen atom shares two of its electrons with the hydrogen atoms to form a stable molecule.

Electronegativity is the measure of an atom's ability to attract shared electrons in a bond. If there is a significant difference in electronegativity between two atoms involved in a bond, then the bond may be either a polar covalent bond or an ionic bond. On the other hand, if there is little or no difference in electronegativity between two atoms, the bond is likely a nonpolar covalent bond. The electronegativity values for oxygen and hydrogen are 3.44 and 2.20, respectively. The difference in electronegativity between these two atoms is 3.44 - 2.20 = 1.24.

Now that we have determined the difference in electronegativity between the oxygen and hydrogen atoms, we can use this information to determine the bond type. The general guidelines for bond types based on electronegativity differences are as follows: 1. If the difference in electronegativity is larger or equal to 1.7, the bond is considered to be an ionic bond. 2. If the difference in electronegativity is between 0.4 and 1.7, the bond is considered to be a polar covalent bond. 3. If the difference in electronegativity is smaller than 0.4, the bond is considered to be a nonpolar covalent bond. Since the difference in electronegativity between oxygen and hydrogen is 1.24 which falls in the range of 0.4 to 1.7, the bond between oxygen and hydrogen in a water molecule is a polar covalent bond. Therefore, the correct answer is: b. a polar covalent bond.