1. The problem asks: What does set B mean in Problem 4? 2. From Problem 4, set B is defined as: $$B = \{\{\{1, 4, 5, 3, 1\}\}\}$$ 3. This means that B is a set containing one element, which itself is a set containing the elements 1, 4, 5, 3, and 1. 4. Note that sets do not count repeated elements multiple times, so the inner set is effectively: $$\{1, 3, 4, 5\}$$ 5. Therefore, B is a set whose only element is the set \(\{1, 3, 4, 5\}\). 6. In simpler terms, B is a set containing one set, and that inner set contains the numbers 1, 3, 4, and 5. 7. This is different from a set that directly contains numbers; B contains a set as its single member. Hence, set B is a singleton set whose only element is the set \(\{1, 3, 4, 5\}\).