Which part of the quadratic formula is under the radical sign?

The correct answer, which is the part of the quadratic formula under the radical sign, is indeed the expression b² - 4ac. This expression is known as the discriminant. The quadratic formula, which is used to find the solutions (or roots) of a quadratic equation in the form ax² + bx + c = 0, is given by:

x = (-b ± √(b² - 4ac)) / (2a)

In this formula, the term under the square root (the radical) determines the nature of the roots of the quadratic equation. If the discriminant b² - 4ac is positive, there are two distinct real roots; if it is zero, there is exactly one real root; and if it is negative, the roots are complex (not real).

The other expressions presented in the choices do not represent the discriminant. For instance, the expression 4a² + b does not have a relation to the determination of the roots of a quadratic equation. Similarly, c² - b and 2ax + b do not relate to the formula used to find the roots, making them not applicable in the context of identifying the part of the quadratic formula that is under the radical sign.