Phospholipids assemble spontaneously into bilayers driven by the attraction of their "tail" portions to each other and of their "head" portions away from each other. the tail regions are:

Answer;

Hydrophobic

Explanation;

-The structure of the phospholipid molecule generally consists of two hydrophobic fatty acid "tails" and a hydrophilic "head" consisting of a phosphate group.

-The lipid bilayer is arranged in two layers of phospholipids with the hydrophilic heads forming the outer edges and the tails forming the interior. In this arrangement, the bilayer has a hydrophobic core that prevents the passage of polar molecules while allowing the relatively free diffusion of non-polar molecules.