JSON

B.

The most compact way to serialize structured data depends on a few factors, including the size of the data, the complexity of the structure, and the requirements of the application. However, in general, the protocol buffer (protobuf) format is known to be the most compact among the given options.

Protobufs are a binary format that encodes structured data in a highly efficient manner, allowing for faster processing and smaller file sizes compared to other formats. They are language and platform-neutral, meaning they can be used to exchange data between different programming languages and environments.

YAML and JSON, on the other hand, are human-readable formats that encode data in a text-based format. YAML is often used for configuration files, while JSON is commonly used in web applications and APIs. Both formats are less compact than protobufs but are easier to read and write, and have good support for various programming languages.

XML is a markup language that encodes data in a hierarchical structure, similar to HTML. It is widely used for exchanging data between different systems and applications, but it is more verbose than protobufs, YAML, and JSON.

Therefore, based on the given options, the most compact format for serializing structured data is protobufs.