Which of the following reactions is a standard formation reaction (i.e. ΔH°f) at t = 298 K?
a. ( H₂(g) + 1/2O₂(g) → H₂O(l) )
b. ( C(graphite) + O₂(g) → CO₂(g) )
c. ( 2C(graphite) + 3H₂(g) → C₂H₆(g) )
d. ( NH₃(g) → 1/2N₂(g) + 3/2H₂(g) )