Answer: a heterogeneous mixture
Explanation: Single phase is defined as the only phase where you cannot see any differentiation in the phases. It will appear as the single form.
A heterogeneous mixture is not a single phase because it is defined as the mixture which contain substances of different phases. Thus there will be more than one phase.
A pure solid is only a single phase or form of solid.
A pure liquid is also consist of only single phase.
A homogeneous mixture is the mixture in which different substances are present but their form is same thus making the mixture a single phase.
