A. Shell environment variables

Shell environment variables are not considered safe because they can be easily modified by the user. For example, a user could set the PATH environment variable to include a malicious directory, which would allow the application to execute arbitrary code.

B. Data received via encrypted network channels

Data received via encrypted network channels is considered more safe than other sources of external input, but it is still not completely safe. The encryption could be broken, or the data could be intercepted and modified before it is decrypted.

C. argv[0] can only have either null or program name

The argv[0] parameter is the name of the program that is being executed. It is not considered a safe source of input because it can be easily modified by the user. For example, a user could change the argv[0] parameter to a malicious program, which would then be executed instead of the intended program.

D. no external input must be trusted

The correct answer is D. No external input must be trusted, regardless of the source. Even if the input comes from a seemingly safe source, it is always possible that the input has been tampered with. Therefore, it is important to always validate and sanitize all external input before it is processed by the application.

The correct answer is therefore D.