The correct option is B Conversion of graphite to diamond
Considering the options one by one,
a) 2O3(g)→3O2(g)
As the number of gaseous moles is increasing, △ng>0
where,
△ng=np−nR
nP= total number of gaseous moles of products
nR= total number of gaseous moles of reactants
⇒△S>0
b) In the conversion of graphite to diamond, the randomness decreases as diamond has a more organised and packed structure than graphite. So, ⇒△S<0