Hydrogen bonds are:

a. the sharing of electrons between a hydrogen atom and another atom.
b. the electrostatic interaction between an atom that has lost an electron and an atom that has gained an electron.
c. the interaction between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom.

The answer to your question is: letter C

Explanation:

a. the sharing of electrons between a hydrogen atom and another atom.  This description is not for hydrogen bonding, in hydrogen bonding atoms do not share electrons, to form this bond.

b. the electrostatic interaction between an atom that has lost an electron and an atom that has gained an electron.  This definition is describing an ionic bonding, not a hydrogen bonding.

c. the interaction between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom. This is the right definition of hydrogen bonding, because is mentioning the interaction between a hydrogen and an electronegative atom.