Which of the following data types is not supported in JSON?

To determine which data type is not supported in JSON, we need to understand the data types that JSON does support. JSON (JavaScript Object Notation) is a lightweight data interchange format that is commonly used for transmitting data between a server and a web application.

JSON supports the following data types:

1. String: A sequence of characters enclosed in double quotes, such as "Hello, World!".
2. Number: A numeric value, which can be an integer or a floating-point number, such as 42 or 3.14.
3. Boolean: A value that can be either true or false.
4. Array: An ordered collection of values enclosed in square brackets, such as [1, 2, 3].
5. Object: An unordered collection of key-value pairs enclosed in curly braces, such as {"name": "John", "age": 25}.
6. Null: A special value representing null or absence of a value.

Based on these supported data types, we can conclude that JSON does not support more complex data types such as:

1. Date: JSON does not have a built-in data type for representing dates. Dates are typically represented as strings in a specific format, such as "2022-01-01".
2. Function: JSON does not support functions as data types. Functions are specific to programming languages and cannot be serialized or transmitted as JSON.
3. Undefined: JSON does not have a data type for representing undefined values. If a value is undefined, it is typically omitted from the JSON structure.

Therefore, the data type that is not supported in JSON is the function data type.