What is the correct classification of a mixture in which both a solid and a liquid are visible?
a. a homogeneous mixture and a solution
b. a homogeneous mixture and a suspension
c. a heterogeneous mixture and a solution
d. a heterogeneous mixture and a suspension

Answer is: D.a heterogeneous mixture and a suspension.

Suspension is a heterogeneous mixture (solute particles do not dissolve), that contains solid particles (for example nickel sulfide or NiS) sufficiently large for sedimentation.  

The internal phase (solid nickel sulfide) is dispersed throughout the external phase (water).

Another example of a suspension is sand in water.