a. This is a steroid ligand since the molecule attaches to a receptor on the outside of the cell membrane.

b. This is a protein ligand since the molecule attaches to a receptor on the outside of the cell membrane.

c. This is a steroid since the shape is of a hydrocarbon ring, which is common in hydrophobic molecules.

d. This is a protein since the molecule is hydrophilic and can cross the phospholipid bilayer easily.