One of the back exercises (13(C)) from the book for practice provided at my school asked whether the following relation is true:
$\Delta_\mathrm{f}H^\circ(\ce{I2,g}) = \Delta_\mathrm{sub}H^\circ[\ce{I2,s}]$ at $\pu{25 °C}.$
I know how to solve the question, but the answer would depend on the meaning of $\Delta_\mathrm{sub}H^\circ[\ce{I2,s}].$ Which of the following reaction does the symbol $\Delta_\mathrm{sub}H^\circ[\ce{I2,s}]$ represent?
$$ \begin{align} \ce{I2(g) &-> I2(s)}\tag{a}\\ \ce{I2(s) &-> I2(g)}\tag{b} \end{align} $$