Pierre Charles L’Enfant’s 1791 plan for Washington, D.C., introduced a pattern of radial streets connecting major locations (e.g., the Capitol, White House), overlaid on a grid, creating a distinctive urban form that influenced American city design. Option C (pattern of radial streets) is the most significant contribution. The APA’sAICP Certification Exam Content Outline(2022), under “Fundamental Planning Knowledge,” states, “L’Enfant’s plan for Washington, D.C., introduced radial streets connecting focal points, a design innovation that shaped U.S. city planning.” Option A (grid pattern) was already common (e.g., Philadelphia). Option B (central open space) was not unique to L’Enfant. Option D (waterfront orientation) was not a primary feature of his plan. TheCPC Study Manual(2024) confirms, “L’Enfant’s radial street pattern was his most influential contribution to U.S. city design,” supporting option C.