its because the propanoic acid's conjugate base is proton deficient. therefore it reacts with the water that is present in the aqueous solution and forms oh- ions, therefore increasing the concentration of oh- in solution, making the ph above 7. its to do with the auto ionisation of water and such but I'm pretty sure thats right. saying its strong base and weak acid isn't sufficient to get more than 1 mark, however i may be over complicating the answer if the question is only worth 1 mark