Which One of These Strings Is the Only Valid JSON

Multiple Choice

Which one of these strings is the only valid JSON string?

A) { "cities": ("New York", "Palo Alto", "Boston") }
B) { "cities": {"New York", "Palo Alto", "Boston"} }
C) [ "cities": {"New York", "Palo Alto", "Boston"} ]
D) { "cities": ["New York", "Palo Alto", "Boston"] }