An imaginary number has which of the following as a definition?

Her is the definition of an imaginary number An imaginary number is a complex number that can be written as a real number multiplied by the imaginary unit i, which is defined by its property i2 = −1. The square of an imaginary number bi is −b2. For example, 5i is an imaginary number, and its square is −25. Zero is considered to be both real and imaginary.
(Got that from google)

The answer to this question is c