Final answer:
In his I Have a Dream speech, Dr. Martin Luther King, Jr. discusses the principle of natural rights, which are inherent and unalienable, as declared in the Declaration of Independence.
Step-by-step explanation:
Dr. Martin Luther King, Jr., in his I Have a Dream speech, referenced a fundamental constitutional principle that is best described as natural rights. These are the rights that the Declaration of Independence declares to be self-evident and unalienable, and include Life, Liberty, and the pursuit of Happiness. This principle supports the idea that these rights are inherent to all people and cannot be justly taken away by government. Therefore, the correct answer to the question posed is D. natural rights.