Final answer:
In a REST API, "extra": null signifies that the key "extra" holds a null value, representing the absence of any value, which is distinct from zero or an empty string.
Step-by-step explanation:
In the context of a REST API, when you encounter "extra": null, it indicates that the "extra" key has a null value. This specifically means that the key has been defined, but no value has been assigned to it, which is different from zero (0) or an empty string.
The correct answer to the question 'The "extra" key value is a ______ value.' is C) null, representing the absence of a value. The "extra" key value is a null value. Null represents the absence of a value, meaning it has no value at all. In JSON, null is a data type that is used to indicate the absence of a value for a particular key.