Answer:
A. (It is the design of this, and of the two succeeding chapters, to describe the prosperous condition of their empire; and after wards, from the death of Marcus Antoninus, to deduce the most important circumstances of its decline and fall; a revolution which will ever be remembered, and is still felt by the nations of the earth.)
Step-by-step explanation:
Edward Gibbon's "The History of the Decline and Fall of the Roman Empire" is a historical work that traces the civilization of the Western empire right from the Roman empire to the decline of the Byzantine empire. This book provides ancient Roman history and gives an insight into the empire through a careful and vast field of study.
As seen in the excerpt given, the main purpose of the text can be seen in the first option. This passage talks about how this chapter, as well as "the two succeeding chapters, [will] describe the prosperous condition of their empire" and also includes the deaths of Marcus Antonius and "deduce the most important circumstances of its decline and fall." It will also include the "revolution which will ever be remembered, and is still felt by the nations of the earth."
Thus, the correct answer is option A.