The sentence in which the underlined word or group of words is a predicate nominative is:
B. Jonas Edward Salk became famous for developing a polio vaccine.
In this sentence, "famous" serves as a predicate nominative, linking the subject "Jonas Edward Salk" with the state of being "famous."
(Note: "famous" is actually an adjective, so if you're looking for an explicit example of a predicate nominative, the correct choice might actually be more about understanding the function of linking verbs rather than a strict definition. None of your provided sentences actually contain an explicit noun serving as a predicate nominative.)
However, the option "C. In the 1940s, polio was widespread" could be interpreted as having "widespread" act similarly as a predicate adjective, rather than a nominative. So please clarify what type of word you are looking for if you need a stricter definition!