Right choice is (b) It is a structure viewer for single platforms
The best I can explain: It is essentially a Swiss-Army knife for structure visualization and modeling because it incorporates so many functions in a small shareware program. It allows display of multiple structures at the same time in different styles, by charge distribution, or by surface accessibility. It can measure distances, angles, and even mutate residues. In addition, it can calculate molecular surface, electrostatic potential, Ramachandran plot, and so on. The homology modeling part includes energy minimization and loop modeling.