Answer:
C). "In the 20th century, St. Louis civic leaders envisioned a memorial that celebrated the city's role in the nation's westward expansion." ("The Gateway Arch," paragraph 1)
Step-by-step explanation:
The piece of evidence which aptly promotes the author's purpose for which he revealed the United States' goal of progress is 'In the 20th century...expansion' from paragraph 1 of 'The Gateway Arch.' The description regarding the civic leaders of St. Louis planning a memorial that could assist in glorifying the city's significance in the westward expansion of the state/nation substantiates that the author reveals it to convey the readers an idea about the symbolic landmarks that were planned to be created for glorifying the achievements of U.S. Hence, option C is the correct answer.