Voronoi Diagram Drawing Variations and Charateristics
The Voronoi diagram of disjoint segments (b) has been thoroughly studied.

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Image from GIS.


See:


  CGAL Manual, Chapter 43:
  2D Segment Voronoi Diagrams. CGAL link.




For the Voronoi diagram of circles (d), see:


  Jin, Li, Donguk Kim, Lisen Mu, Deok-Soo Kim, and Shi-Min Hu. "A sweepline algorithm for Euclidean Voronoi diagram of circles." Computer-Aided Design 38, no. 3 (2006): 260-272. Journal link.
  
  Huber, Stefan. Computation of Voronoi diagrams of circular arcs and straight lines. Magisterarbeit, 2008.


To quote the latter: "Lemma 1.4. The bisector between two circles (...) consists of ellipses and hyperbolas." 
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