Final answer:
The helix-loop-helix is a protein structure motif present in both eukaryotic and prokaryotic proteins, playing a role in gene regulation. Therefore, the correct option is C.
Step-by-step explanation:
The helix-loop-helix is a structural motif in proteins and is part of both eukaryotic proteins and prokaryotic proteins. This motif is involved in the regulation of gene expression and is characterized by two alpha-helices connected by a loop. The helix-loop-helix motif can be found in proteins such as transcription factors that are important for cellular processes in both types of organisms. Therefore, the correct option is C.