Command to turn off variable substitution is '!set variable_substitution=false'.

• A. Not applicable
• B. Unknown
• C. True
• D. False

Answer: (C)

Variable substitution is the process of replacing placeholders with actual values during processing. To control it, you adjust a toggle like variable_substitution. The command shown uses a directive (often seen as a notebook or shell magic) to assign false to that toggle. Setting variable_substitution to false disables the substitution, so placeholders are left as-is. That’s why this statement is true: it correctly turns off variable substitution. If you set it to true, substitutions would be enabled again. The other options don’t describe a real, meaningful status for this configuration.