1. **Problem:** Identify which of the given sets is a null set. 2. **Recall:** A null set (empty set) is a set with no elements. 3. **Analyze each option:** a) {x : x is a natural number less than 0} — Natural numbers start from 1, so no natural number is less than 0. This set is empty. b) {x : x is an even prime number} — The only even prime number is 2, so this set is {2}. c) {x : x is a factor of 2} — Factors of 2 are {1, 2}, so not empty. d) {x : x is a month in a year} — There are 12 months, so not empty. 4. **Conclusion:** The null set is option a). **Final answer:** a) {x : x is a natural number less than 0}