The question asks, "Which describes a number that cannot be irrational?" Let's break down the options provided:

1. A number that represents the length of the diagonal of a square: The length of the diagonal of a square can be expressed using the Pythagorean theorem as $\sqrt{2}$ (for a unit square), which is an irrational number.

2. A number that represents the ratio of the circumference to the diameter of a circle: This ratio is known as $\pi$, which is an irrational number.

3. A number that can be used to solve an algebraic equation: Numbers used in algebraic equations can be either rational or irrational. So, this doesn't necessarily avoid irrational numbers.

4. A number that can be written as the ratio of two integers: This is the definition of a rational number. Rational numbers are by definition not irrational.

Correct Answer:

The correct description is "a number that can be written as the ratio of two integers", as these numbers are rational and cannot be irrational.

Would you like further explanation or details on rational and irrational numbers?

Related Questions:

1. What are the differences between rational and irrational numbers?
2. Can a square root ever be a rational number?
3. How is $\pi$ classified as an irrational number?
4. What is the significance of the number $e$ in mathematics, and is it rational?
5. How do irrational numbers differ when represented as decimals?

Tip:

Rational numbers can always be expressed as terminating or repeating decimals, whereas irrational numbers have non-repeating, non-terminating decimal expansions.