Final answer:
The union of sets A and B, A U B, is {cats, rabbits, fish, dogs}, which contains all unique elements from both sets.
Step-by-step explanation:
The question requires us to find the union (A U B) of two sets A and B, where A includes cats, rabbits, fish and B includes dogs, cats. The union of two sets is a set containing all the elements of both sets, without any duplicates. Therefore, the union A U B will include every distinct element that appears in either set A or set B. That means the answer will be all the unique animals listed in both sets.
To calculate A U B, we combine the elements of both sets: {cats, rabbits, fish, dogs, cats}. As 'cats' is present in both sets, it is only counted once. Thus the union of sets A and B is: {cats, rabbits, fish, dogs}.