B

At the resting membrane potential there is (by definition) no net transmembrane current, so the ion whose equilibrium (Nernst) potential is closest to the RMP will have essentially zero driving force. Chloride is usually passively distributed; its equilibrium potential therefore sits very close to the resting value [Urazaev, 1987, PMID 3448493]. Among the choices, –20 mV is the value nearest to the given RMP of –30 mV, making it the best estimate for Z.