What type of biomolecule is a major part of the cell membrane because it creates a thin hydrophobic layer that maintain selective permeability?

Lipids

Explanation:

The cell membrane is comprised of a bilayer of phospholipids. Phospholipids are the lipids with the phosphate head group. These phospholipids are amphipathic in nature. The head group of phospholipids is hydrophilic while the tails are hydrophobic in nature. The hydrophobic tails of two phospholipid layers form the inner core of the membrane. Their head groups are oriented towards the outer side.  

The hydrophobic core of membrane allows only certain hydrophobic, small and uncharged molecules to pass through it. On the other hand, the large and charged polar particles can not cross the membrane making it selectively permeable.