To classify the soil based on the characteristics provided—strong structure, slickensides, and a high Plasticity Index (PI)—the most suitable option would likely be:
b) Sterkspruit soil form
Sterkspruit soils are known for having slickensides, which are indicative of clayey soils that exhibit shrink-swell behavior, and they typically have a high plasticity index. The other soil forms listed do not typically exhibit both slickensides and a high PI to the same extent.
So the correct answer is: b) Sterkspruit soil form