The primary driving force for natural convection is (a) shear stress forces (b) buoyancy forces (c) pressure forces (d) surface tension forces (e) none of them

Natural convection occurs when fluid motion results from density differences due to variations in temperature, typically in the presence of a gravitational field. The fluid moves due to buoyancy forces, which arise from the variations in fluid density.

Now, we will examine each answer choice to see if it corresponds to the primary driving force for natural convection: (a) Shear stress forces: These forces are usually the result of a fluid moving against a solid surface, and they are not directly involved in natural convection. (b) Buoyancy forces: These forces arise due to density differences in fluids, which cause fluid movement in natural convection. (c) Pressure forces: Although pressure variations can arise as a result of buoyancy forces, they are not the primary driving force for natural convection. (d) Surface tension forces: These forces are the result of a liquid's molecular properties, and they do not typically drive natural convection. (e) None of them: Since buoyancy forces are mentioned earlier in the list, this answer choice is incorrect.

Based on our evaluations, it is clear that the primary driving force for natural convection is buoyancy forces. Therefore, the correct answer is (b) buoyancy forces.