Mice's. In this case the answer needs to indicate that around the attic were many nests that belong to the mice. The rule of thumb here is to add an apostrophe and and 's' at the end of the noun. Therefore the possessive form mice's described that the many nests(plural) belonged to the mice. Mices' would be incorrect because it would suggest that mices is the plural and adding the apostrophe would make it possessive.
