Final answer:
The possessive error in the sentence is 'Salks', which should be 'Salk's' to correctly indicate possession by Jonas Salk.
Step-by-step explanation:
In the sentence “In 1957 Jonas Salk introduced a vaccine for polio. Salks revolutionary contribution to medicine has allowed countless people to live longer, healthier lives,” the possessive error is “Salks.” This should be “Salk's” to show that the revolutionary contribution belongs to Jonas Salk. Therefore, the corrected sentence should read: “In 1957 Jonas Salk introduced a vaccine for polio. Salk's revolutionary contribution to medicine has allowed countless people to live longer, healthier lives.” There are no plural errors in the sentence provided.