Which of the following describes a simple fraction?

A simple fraction refers to a fraction where the numerator (the top number) is less than the denominator (the bottom number), making it a proper fraction. This means that its value is less than 1. When a fraction meets this criterion, it is considered not top heavy or non-improper. The term "top heavy" typically refers to an improper fraction, where the numerator is equal to or greater than the denominator, resulting in a value of 1 or more.

In the context of the other options, a fraction with a numerator of 0 represents 0, which is indeed a valid fraction, but it does not fit the typical description of a simple fraction where the numerator is compared to the denominator's value. Similarly, a negative denominator does not define the fraction as simple or complex; it simply alters the sign of the fraction. Therefore, the correct choice highlights the defining characteristic of a simple fraction that is not top heavy.