Part c - how does phospholipid structure relate to the selective permeability of the plasma membrane?

One of the reasons the plasma membrane can have selective permeability it's because it's composed by a bi-layer of phospholipids.

The interaction between them and other molecules will influence how these molecules will cross the membrane. If the molecule it's hydrophobic it can easily cross, if it's polar molecules and ions (hydrophilic), they cannot cross easily.