The right answer is phospholipids.
Phospholipids are the essential constituents of cell membranes where, with cholesterol molecules, they are organized into a lipid bilayer in which the hydrophobic tails are oriented towards the interior of the structure whereas the polar heads form the two surfaces of the bilayer .
It is the basis of living structures, which are always organized from a cell membrane separating the inside of the cell from the outside environment. Internal membranes can define inner compartments within eukaryotic cells - called organelles - such as mitochondria, chloroplasts, one (or more) nucleus (x), an endoplasmic reticulum, the Golgi apparatus, and so on.