The correct option is Exactly one trigger.

Azure Functions require that each function declares a single trigger which is the event source that starts the function. You can choose different trigger types such as HTTP, Timer, or Blob and you can add input and output bindings for additional data interaction while still having only one trigger per function.

No upper limit on triggers per function is incorrect because the function model does not support multiple triggers on the same function and it enforces a single trigger to start execution.

A fixed cap of 64 triggers per function is incorrect because there is no mechanism to attach multiple triggers up to a numeric cap since functions are defined with exactly one trigger.

Zero triggers or a single trigger is incorrect because a function must have a trigger to be invoked so declaring zero triggers is not a valid function definition in Azure Functions.

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