Which of the following is an empty set?
Answer: C
An empty set, or null set, is a set containing no elements.
A) {x : x² = 9} = {-3, 3}. Not empty.
B) {x : x is an even prime number} = {2}. Not empty (it's a singleton set).
C) {x : x is a month with 32 days}. No month has 32 days. This set is empty.
D) {0}. This is a singleton set containing the element 0. It is not empty.