How should the risk of harm be evaluated in a research study?

The correct choice evaluates the risk of harm in a research study by considering both the magnitude and the probability of harm. This dual approach allows researchers to assess not only how severe the potential harm could be (magnitude) but also how likely it is to occur (probability).

A comprehensive evaluation of risks is critical ethical practice, especially in research involving vulnerable populations, such as prisoners. By analyzing the magnitude, researchers can understand the potential impacts on participants' well-being, while assessing the probability helps to weigh whether those impacts are likely to affect participants in real scenarios. Together, these factors provide a nuanced understanding of the risks that participants may face, which can inform decisions about whether to proceed with the study or how to mitigate identified risks.

The other options center on single aspects that do not provide a holistic risk assessment. For instance, financial cost, duration, or sheer numbers of participants do not give insight into the actual risks posed to participants, emphasizing why a multifaceted evaluation of risk is essential in research ethics.