ANS:C - both (a) and (b)

The behaviour of a mass of fluidised solid depends mainly on both (a) and (b): the fluid characteristics and the particle size.

Explanation:

1. Fluid Characteristics:
   • The properties of the fluid, such as its velocity, density, and viscosity, are crucial in determining the fluidisation behaviour. The fluid must flow through the bed of particles at a sufficient velocity to lift and suspend the particles, creating a fluidised state.
2. Particle Size:
   • The size and distribution of the solid particles significantly affect how they interact with the fluid and with each other. Smaller particles are easier to fluidise and tend to stay suspended with lower fluid velocities, whereas larger particles require higher velocities.