The correct answer is option 'C': By file extension. Let's explore why this is the most common way to discover a media type.

File Extensions:
A file extension is a set of characters that appear at the end of a file name and are separated by a period (e.g., .txt, .jpg, .mp3). These extensions provide valuable information about the type of file and the program or software that can open it.

Media Types:
Media types refer to different types of files, such as text documents, images, audio files, videos, etc. Each media type requires specific software or applications to be opened and viewed or played correctly.

Discovering Media Type:
Discovering the media type of a file is crucial as it helps determine the appropriate software or application to use. There are various ways to discover the media type, but the most common and reliable method is by examining the file extension.

Why File Extension is the Most Common Way:
1. Accessibility: File extensions are easily accessible and visible in the file name. By looking at the extension, users can quickly identify the media type without opening the file.
2. Standardization: File extensions are standardized across different operating systems and platforms. This standardization ensures compatibility and consistency in recognizing media types.
3. Widespread Knowledge: People are generally familiar with common file extensions such as .txt for text files, .jpg or .png for images, .mp3 for audio files, and .mp4 for videos. This familiarity makes it easier to recognize the media type based on the extension.
4. Software Association: File extensions are associated with specific software or applications. For example, .docx files are associated with Microsoft Word, .xlsx with Microsoft Excel, and .pdf with Adobe Acrobat Reader. This association allows the operating system to open the file with the appropriate software automatically.
5. Quick Identification: When browsing files or searching for specific media types, users can identify the desired type by simply looking at the file extension. This saves time and effort in manually inspecting each file's content.

Conclusion:
In conclusion, the most common way to discover a media type is by examining the file extension. File extensions provide accessibility, standardization, widespread knowledge, software association, and quick identification, making them the go-to method for determining the media type of a file.