Final answer:
In the given sentence, there are four prepositions: 'by', 'as', 'off', and 'on'.
Step-by-step explanation:
The question asks to identify the number of prepositions in the given sentence. A preposition is a word that shows the relationship between a noun or pronoun and other words in a sentence. In the sentence, the prepositions are as follows: 'by' shows the relationship between the story and who wrote it (Kate Chopin), 'as' shows the relationship in terms of time between reading and riding, and 'off' and 'on' are showing the relationship of the speaker's position related to the train. Therefore, there are a total of four prepositions.