Answer: D. fluorine
Fluorine is the most electronegative element in the world. Electronegativity is the ability of atoms to attract electrons in a covalent bond. Elements with high electronegativity will attract shared electrons in a covalent bond more strongly.
Electronegativity of elements decreases down the group in a periodic table and also decreases across a period. Generally, elements in the halogen groups (fluorine, chlorine, bromine, and iodine) are known to be very electronegative with fluorine leading the park. In the options listed, fluorine is the most electronegative followed by Chlorine, then carbon, and finally hydrogen.