Question

Which phrase in Martin Luther King, Jr.’s "I Have a Dream" speech is an example of an allusion?

Answer 1

The correct answer is C. Let freedom ring from every hill.

An allusion is a figure of speech which refers to something from an external point of view. It is left to the reader to make a connection between the two things. 
Martin Luther King, Jr., in his speeh "I Have  a Dream" approches the audience's sence of nationalism to inspire them to achieve freedom and liberty.