Which ActionScript 3.0 data type is used to represent a missing value?

The choice of "Null" as the correct answer is based on its specific role in ActionScript 3.0, as in many programming languages. It is used to signify the absence of a value or an intentional assignment of a void state to a variable. When "null" is assigned to a variable, it indicates that the variable is defined but not currently holding a meaningful or usable value.

In contrast, "Number" represents numeric data, which requires a defined value, such as zero or any other number. A variable assigned a "Number" type cannot indicate a lack of a value but merely holds a numeric representation.

"String" is a data type used for textual data. Even an empty string is still considered a value, as it is a defined state (just without any characters), which distinguishes it from "null."

Lastly, the "Boolean" type represents truth values: true or false. It communicates binary states rather than the absence of any value, which further emphasizes why "null" is the appropriate choice for indicating a missing value in this context.

Thus, in ActionScript 3.0, "null" is the designated data type for representing situations where a value is not present, making it the correct answer.