What type of data structure allows for accessible key-value pairs?

The correct answer pertains to the concept of message extras, which are often used in the context of APIs and mobile applications to facilitate the passing of key-value pairs. Message extras are a flexible data structure where you can store data in pairs, making it easy to associate specific keys with associated values. This enables developers to access data efficiently by referencing the key to retrieve the corresponding value.

In contrast, other options like databases, arrays, and lists do not inherently operate on a key-value pair basis. A database is a structured collection of data, typically involving tables and rows, but it can represent key-value pairs in some contexts; however, it is not exclusively designed for that. Arrays and lists are primarily indexed collections of items, allowing access to elements by their position rather than a key, which makes them unsuited for direct key-value access. Thus, message extras stand out as the data structure specifically designed for this purpose, allowing for quick and efficient data retrieval through clearly defined key-value associations.