You are answering a USMLE-style medical question. Question: A microbiologist has isolated several colonies of Escherichia coli on an agar plate. The microbiologist exposes the agar plate to ciprofloxacin, which destroys all of the bacteria except for one surviving colony. Which of the following is the most likely mechanism of antibiotic resistance in the surviving colony? Options: A) Replacement of D-alanine B) Mutation of DNA gyrase C) Inactivation by bacterial transferase D) Modification of surface binding proteins Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.