Explanation

An improper fraction is a fraction in which the numerator is greater than the denominator. Let's evaluate each fraction in the list:

1. $frac{15}{7}$: 15 is greater than 7, so this fraction is improper.
2. $frac{14}{9}$: 14 is greater than 9, which is also an improper fraction.
3. $frac{13}{7}$: 13 is greater than 7, so this is also an improper fraction.
4. $frac{12}{8}$: 12 is greater than 8, so this is also an improper fraction.

Since all four fractions have numerators greater than their denominators, all of them are improper fractions: