Sections in this chapter started with closed sets and open sets, which sound fairly straight-forward, but really they covered a formalisation of how sets are closed or open under a distance metric. eg, a set in two dimensions may be closed conventionally, but may not be closed under an unusual distance metric.
Next up were closures, interiors, boundaries, and the size of sets. Again, normally this would be fairly simple. -less so when looking at these in terms of the distance metric.
This chapter also included a little bonus seventh section introducing topology which looked great (and I've always wanted to learn more about) but unfortunately this section was not assessed, and I didn't have the time for extra reading. It seems this degree isn't just about learning maths, but specifically about learning maths fast.
So that brings Book D to a close!