What form do complex numbers take?

Complex numbers are expressed in the standard form of a + bi, where 'a' represents the real part and 'b' represents the imaginary part multiplied by 'i', the imaginary unit. The 'i' stands for the square root of -1 and is a fundamental concept in complex number theory.

In this context, 'a' can be any real number, while 'b' can also be any real number, and the inclusion of 'i' indicates that 'b' contributes to the imaginary component of the complex number. This representation allows for the effective combination of both real and imaginary parts, leading to the comprehensive handling of numbers that extend beyond traditional real values.

The other forms presented do not adequately represent complex numbers. For example, formats that lack 'i' do not account for the imaginary component, which is critical to the definition of complex numbers. Hence, the correct expression captures the essential characteristics of complex numbers, making it the appropriate choice.