Who are the “men of goodwill” that gladden describes?

The correct answer is "Reformers."

The "men of goodwill" that Gladden describes are the Reformers.

Washington Gladden (1836-1918) was an American politician and social reformer that supported the Social Gospel, in that all Christians must do something to help society, to fight corruption practices and injustices. He wanted to apply Christian teachings to social problems. He wrote the books "Social Salvation" and "Applied Christianity." He was the one to refer the Reformers as "the "men of goodwill."

the answer to your question is a. the salt plains

in 1811, he led an expedition, known as the george c. sibley expedition, to improve relations with the pawnee and kansa tribes, and also to locate the rumored jefferson's salt mountain