Final answer:
The highest percentage of the U.S. population identifies with Protestantism as their self-described religious affiliation, followed by other large groups including Catholics and 'nones'.
Step-by-step explanation:
When considering what self-described religious affiliation is identified by the highest percentage of the population in the U.S., the most prevalent affiliation is Protestant. Protestantism encompasses various denominations within the Christian Church, some of which include Methodist and Seventh-day Adventist. These denominations differ from the Catholic Church, which, while significant in numbers, does not surpass the combined population of various Protestant denominations. The non-religious group, often referred to as 'nones,' is also notable for its sizable and growing presence in American society. However, based on the Pew Research Center's data, Protestantism remains the most substantial self-described religious affiliation in the United States as of the latest available studies.