Which of the following represents pure capacitive reactance of 100 ohms in rectangular notation?
Why Impedance in rectangular form is written R + jX, where the real part is resistance and the imaginary part is reactance. A pure reactance has no resistive part, so R = 0. By convention capacitive reactance is negative and inductive reactance is positive, so 100 ohms of pure capacitive reactance is 0 - j100.
Watch out The choice 0 + j100 has the right magnitude but the positive sign marks it as inductive reactance, not capacitive. The choices with 100 + j0 or 100 - j0 are 100 ohms of pure resistance with no reactance at all.
Capacitors are negative: C comes before L, and minus comes before plus. Real part first, so pure reactance starts with 0.