McDonald's introduced its breakfast menu nationwide in 1977.
The year 1977 marks the national introduction of McDonald's full U.S. breakfast line, not the first appearance of every breakfast item. The menu included the Egg McMuffin along with hotcakes, toasted English muffins, scrambled eggs, sausage, hash browns, and Danish pastries.
The Egg McMuffin came earlier. Herb Peterson developed the sandwich in California, and McDonald's began testing it in 1972. Its combination of a griddled English muffin, egg, Canadian bacon, and cheese was inspired by eggs Benedict. The Egg McMuffin's national rollout was completed by 1975, two years before the broader breakfast menu became official nationwide.
That timeline explains why 1972 and 1975 can appear in histories of McDonald's breakfast, but neither is the best answer to this wording. The question asks about the nationwide breakfast menu as a complete line, which McDonald's dates to 1977. Breakfast later became a major part of the chain's business and helped establish fast-food breakfast as a mainstream category.