Final answer:
Sentence C best supports the notion that the emergent consumer culture changed American identity, impacting what society valued in individuals and influencing aspects of life such as employment and family.
Step-by-step explanation:
The emergent consumer culture at the turn of the century changed what it meant to be "American" by shifting from a focus on individual character and self-reliance to a celebration of consumerism, personality, and the influence of data presented through various media. Sentence C from the provided text options supports this assertion best: "Carnegie's journey reflected a cultural evolution that reached a tipping point around the turn of the twentieth century, changing forever who we are and whom we admire, how we act at job interviews and what we look for in an employee, how we court our mates and raise our children." This sentence encapsulates how the advent of consumer culture influenced various aspects of American life, from professional to personal.