What term describes a file used to conceal classified information inside an MP3 file?

The term that describes a file used to conceal classified information inside an MP3 file is the carrier file. In the context of steganography, a carrier file is the medium that holds the hidden data, which can be in different formats such as images, audio files, or videos.

When an MP3 file is used to hide information, the MP3 functions as the carrier that allows the concealed data to remain undetected. The advantage of using a carrier file like an MP3 is that it is commonly shared and used, making the hidden information less likely to be discovered during analysis.

In this context, while the other options may pertain to different aspects of data concealment or may describe various roles within steganographic practices, they do not specifically represent the concept of a file that serves as a vessel for embedding hidden information. A stegoimage, for instance, relates specifically to images as a carrier, and a payload usually refers to the data being hidden, not the file that carries it.