Which urban planning idea was developed by Kiyonori Kikutake as an alternative to land reclamation?

Kiyonori Kikutake, a prominent member of the Metabolism movement, introduced the concept of Marine City as an innovative urban planning proposal. This idea emerged in response to the increasing challenges of urbanization and land reclamation, particularly in densely populated areas like Japan.

Marine City represents a futuristic conception of floating urban developments that could exist on the ocean's surface, thus alleviating the pressure on land-based resources. By advocating for structures that are not only self-sustaining but also adaptable to the marine environment, Kikutake aimed to redefine urban living in harmony with nature, challenging the traditional notions of urban expansion through reclaimed land.

The other options pertain to different urban planning concepts. The Neighborhood Unit focuses on creating self-contained communities and urban development patterns that emphasize pedestrian accessibility and community interaction. Barbican Development refers specifically to a regeneration project in London that aimed to integrate residential, commercial, and cultural spaces, rather than addressing the broader concept of urban expansion into marine environments. Metabolism, while influential and encompassing ideas around organic growth and adaptability of structures, represents a broader theoretical approach encompassing various urban designs, of which Marine City is a specific example. Thus, Marine City is the most accurate answer in connection to Kikutake's innovative response