Final answer:
During the Progressive Era, Lincoln Steffens, Ida Tarbell, and Upton Sinclair were all writers who exposed wrongdoing as muckrakers. Mark Twain, although a critical writer, was not known for the investigative journalism characteristic of the muckrakers of that time.
Step-by-step explanation:
The question 'All of the following were writers who exposed wrongdoing in the Progressive Era EXCEPT: a. Lincoln Steffens b. Ida Tarbell c. Upton Sinclair d. Mark Twain' seeks to identify which of the listed authors did not work as a muckraker exposing issues during the Progressive Era. Notably, Lincoln Steffens exposed corruption in city politics, Ida Tarbell is famed for her critical investigation of Standard Oil's monopolistic practices, and Upton Sinclair highlighted the brutal conditions of the meatpacking industry in his novel 'The Jungle.' Unlike these three, Mark Twain did not participate in this style of journalism; he was more known for his works of satire and commentary on society that, although critical, did not fit the profile of 'muckraking' during the Progressive Era.