Postmodernism is generally associated with the period after modernism.
In literary history, the postmodern period is often considered to begin around the 1960s.
So statement A is correct.
Statement B is incorrect because postmodernism did not come immediately after World War I.
The period after World War I is more strongly connected with modernism.
Statement C is correct because John Barth, Cormac McCarthy, and Don DeLillo are important writers associated with postmodern fiction.
Statement D is incorrect because Jacob's Room is a novel by Virginia Woolf, not Samuel Beckett.
Statement E is also incorrect because “Sailing to Byzantium” is a poem by W.B. Yeats, not T.S. Eliot, and it is not a postmodernist work.
Therefore, the correct statements are:
\[
A \text{ and } C \text{ only}.
\]