Answer is (a) AgNO3 in NH3
Propyne reacts with AgNO3 in NH3 to give white ppt. of silver acetylide and propene does not react with it. Thus, it is used to distinguish between propene and propyne.
The reaction is as follows :
On the other hand, both of them decolourise alk. KMnO4 and Br2 in CCl4 and both of them give addition reaction with conc. H2SO4