Final answer:
To make x the subject of the formula, x is already isolated in the given equation x = (23 / vd) * Za. Among the provided options, the correct representation is b. x = 23Za / vd.
Step-by-step explanation:
To make x the subject of the formula, we need to isolate x on one side of the equation. Starting from the given formula x = (23 / vd) * Za, it already appears that x is the subject.
However, if we are considering options a through d where the equation is possibly rearranged, only one of them correctly represents the given equation.
The correct option would be b. x = 23Za / vd, as it maintains the original relationship between variables.