Final answer:
The person most likely to stop and help a stranger due to the bystander effect is someone who is alone, because they do not experience the diffusion of responsibility that occurs in larger groups.
Step-by-step explanation:
According to research on the bystander effect, the person most likely to stop and help a stranger is b) Someone alone. This phenomenon occurs due to the diffusion of responsibility, where individuals in a large group are less likely to help because they believe someone else will take action. Additionally, someone who is alone does not experience this diffusion and therefore may feel a greater personal responsibility to intervene. A person in a hurry (c) is less likely to stop due to time constraints. Although someone with a medical background (d) might be more equipped to help, the presence of a large group can still lessen their likelihood of intervening as a result of the bystander effect.