A substitution steganography uses this part of a carrier file.

Substitution steganography involves embedding a secret message within a carrier file by replacing the least significant bits of the carrier's data. This technique takes advantage of the fact that changes made to the least significant bits will have a negligible impact on the overall appearance or function of the carrier file, which is typically an image, audio, or video file. By altering the most insignificant portions, the hidden message can be concealed without significantly altering the visual or auditory characteristics of the carrier, making it difficult to detect.

Using the least significant bits is effective because they contribute the least to the value of the pixel or data point. For example, in digital images, the least significant bit of a pixel value can change the color shade very slightly, often imperceptibly to the human eye. Thus, this method allows for efficient data hiding while maintaining the integrity of the carrier file's appearance or sound quality.

The other options refer to aspects of the carrier that would either result in noticeable alterations when modified or are not directly relevant for embedding the secret message in a steganographic context.