19 and 23 are both prime numbers, which means they only have 2 factors, 1 and themselves. Since 21 and 25 are composite numbers, those two answers are both incorrect.
We now have 21 and 25. 25 has three factors: 1,5, and 25. Even though 25 equals 5^2, the factor only counts once. 21 has 4 factors: 1, 3, 7, and 21. This is four factors, which is more than 3.
Final answer: 21
