Which amongst the following species is least basic?

Step-by-step solution

Ethylamine is an aliphatic amine with a localized lone pair on nitrogen, making it the most basic among the given options. Aniline is an aromatic amine where the lone pair is delocalized into the benzene ring, reducing its basicity compared to aliphatic amines. Acetamide is an amide where the lone pair on nitrogen is strongly delocalized into the carbonyl oxygen via resonance, making it a very weak base. Protonated acetamide is the conjugate acid of acetamide. Since it has already accepted a proton and carries a po