What is the effect on the magnitude of pure reactance when it is converted to susceptance?
Why Susceptance is the reciprocal of reactance, just as conductance is the reciprocal of resistance. For a pure reactance, B = -1/X, so a 50 ohm reactance becomes 0.02 siemens of susceptance. The question asks about magnitude, and taking the reciprocal is what changes the number's size.
Watch out The choice about reversing the sign describes what happens to the sign in B = -1/X (an inductive reactance gives negative susceptance), but that is not a change in magnitude.
Susceptance is reactance flipped over: B = -1/X. Magnitude reciprocal, sign flipped.