Final answer:
The nurse should respond with option b. 'You cannot assume the safety or efficacy because there is no single standard way of measuring either aspect.'
Step-by-step explanation:
The nurse should respond with option b. 'You cannot assume the safety or efficacy because there is no single standard way of measuring either aspect.' Even if an herb contains a standard amount of extract, this does not guarantee its safety or efficacy. Safety and efficacy are complex and multifactorial factors that require rigorous testing and evaluation.