The phrase "subsequent secession" as used in the context of the Pony Express indicates a soon to come separation, as expressed in option D and explained below.
The adjective "subsequent" is used to indicate sequence, referrring to something that comes after something else in time. For example, if there is a party after a ceremony, we can say: "There is a ceremony and a subsquent party."
The noun "secession" means the same as separation or withdrawal. Therefore, a "subsquente secession" is a separation that took place after another event.
The paragraph discusses the events that led to the Civil War in the United States. As we know, the radical southern states, as mentioned in the paragraph, tried to separate themselves from the rest of the country.
With the information above in mind, we can choose option D as the correct answer. "Subsequent secession" means a soon to come separation.
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