Capulet: what noise is this? give me my long sword! lady capulet: a crutch, a crutch! why call you for a sword? capulet: my sword, i say! old montague is come, and flourishes his blade in spite of me. what is the purpose of this dialogue? a. to foreshadow the tragedy b. to describe the setting c. to provide comic relief d. to build suspense

To provide comic relief is the correct choice. In these lines, what is basically happening is that old men are just itching to fight one another. Lady Capulet is the only one who seemingly acknowledges the fact that both of the heads of the houses are too old to fight with swords, and jokingly points out that "a sword? what you need is a crutch." 

If you have read the entire play, this does not foreshadow the terrible tragedy of the fall of Romeo and Juliet. This is about Capulet and Montague, and neither end up dying or fighting each other with swords by the end of the play. This also does not tell you anything about the setting of the story, and from these lines, I would not even realize that they were simply at a party. This also does not help build much suspense, even though this looks like an acceptable answer because it is not building up to anything. Lady Capulet immediately shoots down his idea to fight Montague with his sword, and they never end up physically fighting later on.