Final answer:
Mark Twain's boyhood, as depicted in his writings, took place along the shores of the Mississippi River, which had a significant influence on his literary works.
Step-by-step explanation:
The answer to the question is A) By the river. In the passage, Mark Twain's boyhood is situated by the river. The passage describes Twain's vivid experiences on a steamboat on the Mississippi River, and his training to pilot the boat. The river and shore scenes symbolize the true expression of American ideals found in nature.
The question relates to American author Mark Twain and the setting of his boyhood as depicted in his literature. Given the information provided, Twain grew up near the banks of the Mississippi River, which was a profound influence on his works.
Considering the details about Twain's experiences aboard a steamboat during his training as a riverboat pilot, and the fact that many of his stories, including The Adventures of Huckleberry Finn and Life on the Mississippi, revolve around the river, it's clear that Twain's boyhood sat by the river. The river scenes in Twain's works are not just backgrounds but serve as a canvas on which he explores and expresses American ideals through the interactions of characters like Huck and Jim with nature and society.