1. **State the problem:** Determine which of the given sets are null sets (empty sets). 2. **Recall the definition:** A null set (empty set) is a set that contains no elements. 3. **Analyze each set:** A) Set of all prime numbers between 15 and 19. - Prime numbers between 15 and 19 are 17 only. - So this set is \{17\}, which is not empty. B) \{x : x < 5, x > 6\} - This set contains elements x such that x is less than 5 and greater than 6 simultaneously. - No number can satisfy both conditions at the same time. - Therefore, this set is empty (null set). C) \{x : |x| < -4, x \in \mathbb{N}\} - The absolute value |x| is always \(\geq 0\). - It cannot be less than -4. - Hence, this set is empty (null set). D) B and C - Both B and C are null sets. 4. **Final answer:** Sets B and C are null sets. $$\boxed{\text{Null sets: B and C}}$$