Which of these is a synthesis or a combination reaction?

a. SO2(g) + H2O(l) H2SO3(aq)
b. 2HgO(s) 2Hg(l) + O2(g)
c. Zn(s) + FeSO4(aq) ZnSO4(aq) + Fe(s)
d. Ca(NO3)2(aq) + 2NaOH(aq) 2NaNO3(aq) + Ca(OH)2(aq)​