Upon which is a phrase commonly used in formal writing to indicate a relationship or connection between two things. However, there are a number of synonyms that can be used in its place, including whereon, whereby, wherein, whereupon, on which, and with respect to which. These alternatives offer more variety and precision in language, enabling writers to better convey their intended meaning. For example, whereupon emphasizes a cause-and-effect relationship, while with respect to which indicates a more abstract connection. Ultimately, the choice of synonym will depend on the specific context and desired emphasis of the sentence.