Which of the following structures is common to all sphingolipids?

Sphingolipids are a class of lipids that are structural components of membranes. Ceramide is a component of sphingolipids. Ceramide is composed of sphingosine, a long-chain amino alcohol with a saturated fatty acid linked to the amino group. Sphingolipids can be differentiated on the basis of the "X" group that is esterified to the terminal hydroxyl group of ceramide. Carnitine is involved in the oxidation of fatty acids. Carnitine is impoant in transferring fatty acids from the cytoplasm into the mitochondria (the carnitine shuttle). Diacylglycerol is the alcohol common to all phospholipids. The second alcohol (e.g., choline, ethanolamine, serine) contributes the polar head that distinguishes the different classes of phospholipids. Like sphingolipids, phospholipids are found in membranes. Sphingomyelin is a sphingolipid with phosphocholine as its "X" group. It is a component of the myelin sheath. Ref: Botham K.M., Mayes P.A. (2011). Chapter 24. Metabolism of Acylglycerols & Sphingolipids. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.