Answer: Base rate
Explanation: Base rate is the rate that determines about the basic or minimum rate that is set for certain population having particular features and it is supposed that nothing is known about the individual apart from the knowing that he/she belongs to the population.
The situation mentioned in the question describes about the base rate situation because even though the rate of shark attacks are low in USA ,a base rate is present for the minimum population facing the shark attacks. Knowing this situation, people avoid swimming activity .