According to Baidu Education, S-R variation refers to (). A. morphological variation. B. drug resistance variation. C. toxicity variation. D. colony variation. E. antigen variation. The correct answer is D. colony variation.

Answer analysis: S-R variation S-to-maintenance, also known as colony variation, refers to the transformation of bacterial colonies from smooth swelling and sliding state to rough and shrinking state, which is called S type and R type.