Intermolecular Forces

Hydrogen bonding occurs when hydrogen is directly bonded to a highly electronegative atom (like oxygen, nitrogen, or fluorine). Let's examine the list: Ethanol: Contains an O-H bond, so it can form hydrogen bonds.
Acetic acid: Contains an O-H bond, so it can form hydrogen bonds.
Ethylamine: Contains an N-H bond, so it can form hydrogen bonds.
Trimethylamine: Contains only N-C bonds. No N-H bond, so no hydrogen bonding.
Salicylic acid: Contains an O-H bond, so it can form hydrogen bonds.
Ethanal: Contains only C-H bonds. No hydrogen bonding.
Ethanol, acetic acid, ethylamine, and salicylic acid can form hydrogen bonds. Thus, there are 4 such substances.