Tricky. As with any multiple choice question, the first thing you want to do is eliminate the answers that are WRONG. In this case (d).
If you don't know the difference between the Declaration of Independence (b) and the Constitution (a) & (c) you are in trouble. However, you should be able to eliminate (b) because that document addresses the colonies as a national entity and does not address individuals.
Freedom of speech is protected by the Constitution (b). And the Bill of Rights (a) is within the Constitution... the first 10 amendments.
The best answer is (a) because the Bill of Rights specifically talks about "individual" rights within the Constitution. Further amendments to the Constitution address state rights and separation of powers among legislative, executive and judicial branches.
The answer (c) is definitely correct because (c) completely includes (a). Perhaps you could say that other rights outlined in the Constitution could bleed into "free speech" and that the entire document of the Constitution outlines the fullest picture of protections in the United States.
The answer choices do not point to one clear answer.