Believe it or not,
Boccaccio was actually heavily influenced by Dante. In the end, they have more similarities than differences. They're stylistically very different. Whereas Dante used terza rima (a rhyme scheme used often in epic poetry), Boccaccio used simple prose. Furthermore, the Divine Comedy is a classic example of a "descent narrative." That is, the protagonist descends into hell, learns something about himself and his place in the world, and returns to the surface (and on to paradise, in this case). In comparison, the Decameron is simply a series of tales told over ten days. Boccaccio was much more "R" rated, so to speak. Dante was, while in many ways more graphic, a much cleaner in terms of language. While Boccaccio centered his satire in "real" situations, Dante used a fantasy of sorts to illustrate his ideas. Boccaccio got most of his stories from medieval folk traditions, while Dante derived much from ancient authors such as Homer and Virgil.
The Decameron is much more humanistic (in the classical sense) than the Divine Comedy, which retains a great deal of medieval philosophy.
There are many ways in which you might compare the two authors.
Both authors wrote in vernacular languages as opposed to Latin. They also wrote about the female ideal personified in a woman: Dante's Beatrice and Boccaccio's Fiammetta.
Boccaccio was much younger than Dante (Dante died in 1321, when Boccaccio was eight years old). Because of this, he served as an inspiration throughout Boccaccio's whole life. In his early works, Boccaccio attempted to emulate Dante.
During his later career, Boccaccio would become more critical of Dante. Nevertheless, he saw <em>The Divine Comedy</em> and <em>The Decameron</em> as being directly connected.
“When thinking” is the right answer (the definition of retrospect is “a survey or review of a past course of events or period of time”. So “when thinking “ is most likely the right answer.