While all your suggestions are grammatically sound and would be interpreted as @Ronald Sole suggests, generally when talking about a location's residents, you often use a derivative of the location name. This is known as a demonym. In your case the sentence would most succinctly be written:

Londoners are happy

I say derivative as while the word often simply involves adding an 'n' to the location for countries (American, Russian, Costa Rican), or 'er' to a town or city (the afore-mentioned Londoner, New Yorker, Berliner), it can get a bit more complex: Germans lose a 'y' from Germany, for a Mexican the 'o' changes to 'an', Canada also adds an 'i' before the 'a' for Canadian.

Sometimes the demonym is just wierd: Norwegian for Norway people is comprehensible, but Glaswegian for Glasgow? A Liverpudlian (because puddles are little pools?) comes from Liverpool, someone from Newcastle is Novacastrian and a resident of Culiacán is, apparantly a Cliché.

Wikipedia has a sizeable list of city demonyms.

While all your suggestions are grammatically sound and would be interpreted as @Ronald Sole suggests, generally when talking about a location's residents, you often use a derivative of the location name. This is known as a demonym. In your case the sentence would most succinctly be written:

Londoners are happy

I say derivative as while the word often simply involves adding an 'n' to the location for countries (American, Russian, Costa Rican), or 'er' to a town or city (the afore-mentioned Londoner, New Yorker, Berliner), it can get a bit more complex: Germans lose a 'y' from Germany, for a Mexican the 'o' changes to 'an', Canada also adds an 'i' before the 'a' for Canadian.

Sometimes the demonym is just wierd: Norwegian for Norway people is comprehensible, but Glaswegian for Glasgow? A Liverpudlian (because puddles are little pools?) comes from Liverpool, someone from Newcastle is Novacastrian and a resident of Culiacán is, apparently a Cliché.

Wikipedia has a sizeable list of city demonyms.