Which of the following is a quadratic equation?

**(A)**

**(B)**

**(C)**

**(D)**

**(E)** Both **(C)** and **(D)**

Quadratic equations are the equations of the form where a, b and c are real numbers and .

is not a quadratic equation because in this case. It is a linear equation.

is not a quadratic equation because in this case. It is also a linear equation.

is not a quadratic equation but it is a quadratic polynomial. To make it a quadratic equation, we will equate it to . It means that is a quadratic equation but is not.

is of the form and . Therefore, it is a quadratic equation.

Hence, the answer is **(D)**.