In milk, phospholipids (phosphatidylcholine, phosphatidylethanolamine, sphingomyelin) are concentrated almost entirely in the Milk Fat Globule Membrane (MFGM). During butter churning, the mechanical impact ruptures the fat globule membranes, and the polar membrane fragments are preferentially washed into the aqueous serum phase. Consequently, buttermilk contains the highest proportion of phospholipids (about $0.15\%$ to $0.20\%$ on a whole liquid basis, and up to $1.5\%$ to $2.0\%$ on a total lipid basis), far exceeding cream, butter, or anhydrous butter oil.

Option analysis:

• Cream: Lower phospholipid proportion relative to total lipids ($0.3\%$ to $0.4\%$).
• Butter: Most phospholipids are lost in the buttermilk during churning ($0.15\%$ to $0.2\%$).
• Butter oil: Minimal, as phospholipid-rich curd is separated during clarifying.
• Butter milk: Correct. Contains the maximum concentration of MFGM-derived polar phospholipids.