The sphingosine backbone is a characteristic of:
Sphingolipids correctly identifies the characteristic feature or property described in this question. In Lipids without annotations, specific characteristics define and distinguish biological molecules, organisms, or processes from one another. The feature described by Sphingolipids is a defining property that arises from its unique molecular structure, evolutionary history, or physiological role. The other options (Phospholipids, Glycerophospholipids, and Sterols) describe characteristics of different entities, represent incorrect properties, or apply to the subject under different conditions.
Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 10