Right answer is (a) Gas or liquid
To explain I would say: The function of the mobile phase in chromatography is to flow over the stationary phase through the packed bed or column. So, only fluids (liquid or gas) can be employed as mobile phase whereas solids cannot be mobile phase, simply due to its inability to flow like fluids and also the interaction with the stationary phase (solid or liquid) will be affected and hence, solids cannot be used as mobile phase.