Final answer:
Catholicism is the denomination with the highest membership in the United States, having more members than any individual Protestant denomination.
Step-by-step explanation:
The denomination with the highest membership in the United States is Catholicism. While there are multiple Protestant denominations in the U.S., including Baptist, Episcopal, Lutheran, Methodist, Pentecostal, and others, and some like the Southern Baptist have experienced substantial growth, the Catholic Church still has more members than any single Protestant denomination. According to Pew Research Center data from 2020, more people identify as Catholic than as members of any individual Protestant denomination, making Catholicism the answer to the question.