Which statement correctly describes the standard Gibbs free energy of formation, ΔGf∘, of an element in its most stable form at 298 K and 1 bar?
a) It equals the element’s standard molar entropy at 298K.
b) It equals the element’s standard molar Gibbs free energy of sublimation.
c) It is assigned a value of zero by definition.
d) It is negative for elements that exist as gases at 298K because gas formation is spontaneous.