Which of the following best defines a rational number?

A rational number is defined as any number that can be expressed in the form of a fraction, where the numerator is an integer and the denominator is a non-zero integer. This definition encompasses not only integers but also fractions themselves. For example, the integers 2 and -3 can be represented as 2/1 and -3/1, respectively, which are both forms of fractions. Therefore, option A directly captures this broad definition, as it asserts that an integer, which is a type of rational number, can indeed be expressed as a fraction.

The other choices do not accurately describe rational numbers. One may think that option B is relevant, but it actually indicates the opposite of a rational number, as rational numbers can always be expressed as a simple fraction. Options C and D further limit the definition incorrectly, as rational numbers include negatives, fractions, and integers, not just positive whole numbers or numbers greater than zero. Thus, option A provides the comprehensive understanding that aligns with the definition of rational numbers.