Answer: foxes' ears
Step-by-step explanation:
When an item doesn't belong to only one person, we use the apostrophe to show plural possession in such case.
It should be noted that "fox's" is the possessive for a single fox while foxes' is the possessive for plural fox. An apostrophe is added at the end of foxes shows that the ears of multiple foxes are being talked of.
Therefore, the correct option is "foxes' ears".