Final answer:
In Norse mythology, according to the Prose Edda, Ymir's flesh was used to create the Earth. This is part of the Norse creation myth.
Step-by-step explanation:
The question refers to the Prose Edda, a collection of Old Norse poems which contains many myths of Norse mythology. According to the Prose Edda, after the giant Ymir was slain by Odin and his brothers, the different parts of Ymir's body were used to create the world. From Ymir's flesh came the Earth, making option 'a' the correct answer to the question. This myth is a significant part of the Norse creation story and represents an ancient view on the formation of the world.