B, Xiao Tiansheng's curly hair and Xiao Liutiansheng's straight hair are the same creature, and the different realization of the same trait is a relative trait, and B is correct;
C. Biological traits are determined by genes. People's height and weight are not determined by genes and cannot be called relative traits. C error;
D Gao Xiao's big eyes and Xiao Lu's tall nose are two characters, not relative characters. D is wrong.
Therefore, choose: B.