Veterinary Biochemistry & Metabolism

$18:2(\Delta^{9,12})$ denotes an 18-carbon polyunsaturated fatty acid with two double bonds at carbons 9 and 12, which is linoleic acid (cis-9, cis-12-octadecadienoic acid). Oleic acid is $18:1(\Delta^9)$ and $\alpha$-linolenic acid is $18:3(\Delta^{9,12,15})$.

At physiological pH ($\sim 7.4$):
Phosphatidylethanolamine carries a negative charge on phosphate and positive on ethanolamine amino group $\implies$ net charge 0 (zwitterionic) (A)
Phosphatidylcholine is also zwitterionic (net charge 0), not cationic (B is false)
Phosphatidylserine has $-1$ (phosphate), $-1$ (carboxyl), $+1$ (amino) $\implies$ net charge $-1$ (anionic) (C)
Cardiolipin has two phosphate groups $\implies$ net charge $-2$ (anionic) (D).

Both statements are true. The protein-to-lipid ratio varies dramatically depending on physiological function (e.g., mitochondrial inner membrane is $\sim 75\%$ protein for transport/respiration, whereas the myelin sheath consists of $\sim 80\%$ lipid and only $\sim 20\%$ protein to provide high electrical insulation along axons). Reason (R) directly explains the lipid-rich nature of myelin (A).