Question

why do phospholipids form a bilayer in water?
the phosphate portions repel each other.
the hydrophilic and hydrophobic parts attract each other.
the phosphate portions attract water, and the lipid portions repel water.
the lipid portions attract water, and the phosphate portions repel water.

Explanation:

Phospholipids have a hydrophilic (water - loving) phosphate head and a hydrophobic (water - fearing) lipid tail. In water, the hydrophilic phosphate portions interact with water (attract water), while the hydrophobic lipid portions try to avoid water (repel water). This leads to the formation of a bilayer where the phosphate heads are on the outside (facing water) and the lipid tails are on the inside (away from water).
The first option is incorrect because phosphate portions do not repel each other in the context of bilayer formation. The second option is wrong as hydrophilic and hydrophobic parts do not attract each other in this way. The fourth option has the properties reversed (lipid portions repel water, phosphate portions attract water).

Answer:

The phosphate portions attract water, and the lipid portions repel water.