The number that can be written in the form a+bi where i^2 = -1.

• A. Real number
• B. Complex number
• C. Imaginary number
• D. Rational number

Answer: (B)

Numbers written as a + bi with i^2 = -1 belong to the set of complex numbers. A complex number has a real part a and an imaginary part bi. If the imaginary part is zero (b = 0), you just have a real number, but it’s still a complex number in this framework. If the real part is zero (a = 0) and b ≠ 0, you have a pure imaginary number, which is also a complex number. Rational numbers are real numbers expressible as a ratio of integers, which is a narrower category. So the form a + bi describes complex numbers, the broadest and most inclusive group.