# Hydrogen bonding is an electrostatic attraction between covalently bonded hydrogen atom of one molecular with highly electronegative atom (N or F or O) of the same molecular or another molecule. or another molecular .It is of two types -(i) Intermolecular hydrogen bonding where molecular association is caused due to interaction of $H-atom$ of one molecular and N or F or 0-atoms of different molecules. (ii) Intramolecular hydrogen bonding where hydrogen bonding occurs between H atom and N or F or O-atom of the same molecule. Intermolecular hydrogen bonding increases the boiling point of the compound and its solubility in water , whereas intramolecular hydrogen bonding decreases the boiling point of the compound and solubility in water.Highest viscosity is exhibited by

$(a)\;\text{Glycerol} \\ (b)\;\text{Ethylene glycol } \\(c)\;\text{Ethanol } \\(d)\;\text{Ammonia}$

The extent of H- bonding is maximum in glycerol due to three $-OH$ groups per molecule.
Hence a is the correct answer.