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The context of this sentence makes clear analogy. The behavior of electrons around the nucleus is a planetary model. That is, electrons orbit the nucleus as the planets orbit the sun in the solar system. To orbit is to revolve around another body in a specific path. To envelop is to cover completely. The electrons do not cover the nucleus. To converge is to move toward a single point. This passage describes the path of a rotation around a point, not a movement toward that point
Since the comparison looks at similarities, the correct choice is analogous. Components which are analogous show likenesses or similarities. Components which are homologous are identical and are derived from shared sources. This term is generally applied to biological elements. Retrogression refers to a return to a prior development stage. There is no mention of evolution or development in this passage.