Read the following excerpt from "I Sell a Dog" by Mark Twain:
I was out of money and I went down to Washington to see if could earn enough there to keep me in bread and butter while I should write the book.
What is the role of the idiom in this sentence?
A. To show that Twain was used to living a hand-to-mouth existence
B. To describe the typical diet of people who lived during Twain's time
C. To add humor to the sentence by equating money with something to eat
D. To indicate that Twain needed money in order to eat

Idioms are the most commonly employed figurative device to propose a sharp and rich language that conveys a meaning distinct from the literal meaning of the words presented. Such a device assists to present a more elaborate and spicier description offering an indirect meaning.

As per the question, the given idiom 'I..........book' functions to indicate that 'Twain required money to eat'. Twain was a realist who aimed to portray the real-life subjects with a touch of subtle elements. In this excerpt from his "I Sell a Dog", he intends to hint that he needed money for 'bread and butter'(eat). Thus, option D is the correct answer.