The characteristic feature of a Rossmann fold is:
Alternating β-strands and α-helices correctly identifies the characteristic feature or property described in this question. In Protein Structure, specific characteristics define and distinguish biological molecules, organisms, or processes from one another. The feature described by Alternating β-strands and α-helices is a defining property that arises from its unique molecular structure, evolutionary history, or physiological role. The other options (Only α-helices, Only β-strands, and A random coil structure) 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. 4