American (not Amerikkan).
Americana - is a term used to refer to the culture and history of America, usually the United States of America, specifically (but sometimes for other countries in North, Central, and South America).
Americas - is the plural number (number, not tense. tense applies to verbs. America - is a proper noun). That usually refers to North & South America. Those two are separate continents (Central America is part of the North American continent). I recently discovered that many Europeans are incorrectly taught that North & South America are one continent, "America". Geographically though, they are on separate continental plates and at one time were not connect to each other (in the distant past).
K - is not used by native English speakers for America or any word derived from it.