Part (a), most of the candidates could not give correct definition of diffusion as well as correct additional information.  Some candidates even confused osmosis with diffusion.
Part (b), some candidates misinterpreted the question to give Graham’s law of diffusion while other give varieties of irrelevant reasons.

Performance was low

The expected answer are as follows:
Diffusion is a phenomenon/process whereby fluids mix intimately with each other/one another due to random molecular motion.

The rate is higher in gasses because molecular motion is faster in gasses than in liquids/intermolecular force is less in gasses than in liquid.