The answer is c. The NNT is calculated from the absolute risk reduction
The number needed to treat (NNT) is the number of patients who need to receive an intervention to achieve one positive outcome (such as a life saved or stroke prevented). It is calculated as the inverse of the absolute risk reduction (ARR): NNT = 1/ARR.
The NNT is an important pharmacoeconomic measure of the effectiveness of healthcare interventions and can be used for assessing the efficacy of treatments of established disease as well as of prophylactic interventions.
The number needed to harm (NNH) is the number of patients who on average need to receive an intervention for one of them to have an adverse outcome (come to harm) because of the intervention. NNT and NNH are both epidemiological measures with similar methodology, but they are numerically unrelated.