A complex number is defined as a number that can be expressed in the form a + bi, where a and b are real numbers and i is the imaginary unit, which satisfies the equation i² = -1.
In the options provided:
- 7, 2, 3, and 4 are all real numbers.
- 9i is a complex number because it can be expressed as 0 + 9i, where the real part is 0 and the imaginary part is 9.
- p is not defined in the context; it does not represent a number with real or imaginary parts.
Thus, the only complex number from the list is 9i.