Which of the following is an example of a transitional word?

A transitional word serves the purpose of connecting ideas and guiding readers through a piece of writing. "Therefore" is a prime example of a transitional word because it is used to indicate a logical conclusion or result based on previously stated information. It helps the reader understand the relationship between ideas, making the text more coherent and easier to follow.

In contrast, other options do not function as transitional words. "Book" is a noun referring to a physical or digital publication, "beautiful" is an adjective that describes something that is visually pleasing, and "quickly" is an adverb that describes the manner in which an action is performed. None of these serve the connective function that transitional words do.