Final answer:
Trick-or-treating in the United States became popular after being featured in children's magazines in the 1940s, influenced by traditions brought by various immigrants, including the Irish.
Step-by-step explanation:
The popularity of trick-or-treating in the United States can be traced to a series of cultural developments and its increasing mention in children's media. The tradition was influenced by various immigrant customs, but it particularly gained momentum after being depicted in children's magazines in the 1940s. The correct answer to the question that best describes when trick-or-treating became popular in the United States is: C. Trick-or-treating became popular after it was described in children's magazines.