An allusion can best be defined as .

1 an explicit statement of fact
2 an unconfirmed piece of information
3 a symbolic statement
4 a subtle indirect reference

D. a subtle indirect reference

Explanation:

An allusion is a reference, from a literary work to another work of fiction, a film, a piece of art, or even a real event. An allusion can be used as a straightforward device to enhance a text by providing further meaning, but it can also be used in a more complex sense to make an ironic comment on one thing by comparing it to something that is dissimilar. It's an expression designed to call something to mind without mentioning it explicitly; an indirect or passing reference.