Because a chemical we will smell needs to diffuse into the fluid surrounding the
olfactory receptors, which of the following substances would not be able to be
detected?
Lipids, proteins, amino acids or sugars

Proteins, amino acids, and sugars  

Explanation:

In order to diffuse into the fluids surrounding the olfactory receptors, the substances must first be in the gas phase.

There exceptions to every rule, but amino acids, proteins, and sugars are  solids with strong intermolecular forces. They have such low vapour pressures that not enough molecules are produced to reach the olfactory threshold.

Lipids are usually less polar and have weaker intermolecular attractive forces. Many of them have an odour.