Which type of document consists only of key-value pairs?

A structured document consists solely of key-value pairs, making it the correct choice. Key-value pairs are a formalized way of storing data where each key (a unique identifier) is associated with a value (the information related to that key). This format is highly organized and facilitates easy data retrieval and processing.

In contrast, unstructured documents do not have a predefined format or organization, which means they can contain text, images, or other data types that are not easily categorized into key-value pairs. Semi-structured documents may include key-value pairs but also contain additional elements or a mix of structures that prevent a purely key-value format. Digital documents simply refer to any document in a digital format and do not imply a specific structure or organization of data.