In plasma membrane, lipid molecules are arranged in

The correct option is head parallel.

To understand why this is the correct arrangement, let us look at the structure and properties of the lipid molecules that make up the plasma membrane:
1. Amphipathic Nature: Phospholipids, the primary lipids in the plasma membrane, are amphipathic molecules. This means they possess both a hydrophilic (water-loving) polar head and hydrophobic (water-fearing) non-polar hydrocarbon tails.
2. Bilayer Formation: When placed in an aqueous environment (like the fluids surrounding and inside cells), these lipid molecules spontaneously organize themselves into a double layer, known as a lipid bilayer, to achieve the most thermodynamically stable state.
3. Parallel Alignment of Heads: In this bilayer, the hydrophobic tails point inward, facing each other in the interior of the membrane to avoid contact with water. The hydrophilic polar heads point outward to face the aqueous environment of the cytoplasm on the inside of the cell and the extracellular fluid on the outside. Consequently, the heads at each surface of the membrane align side-by-side, parallel to the plane of the membrane surface. This is referred to as the head parallel arrangement.

Other arrangements like alternate, scattered, or series would expose the hydrophobic tails to the aqueous environment, which is energetically unfavorable and would disrupt the stability and barrier function of the plasma membrane.