Why cannot outside programs determine the existence of malicious code with 100 percent accuracy?
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Users do not update their scanners frequently enough
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Firewalls are not intended to detect malicious code.
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The purpose of a string depends upon the context in which it is interpreted
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The sourced code language is often unknown.
Static analysis and antivirus scanners cannot achieve 100% accuracy because whether a code string is malicious depends entirely on how it's interpreted in a specific runtime context. The same sequence of bytes could be harmless data in one context but executable malicious code in another. Polymorphic malware and code obfuscation techniques exploit this fundamental limitation - scanners examine static patterns but cannot predict all possible runtime interpretations without executing the code (which is unsafe). Options A and B describe practical limitations, while D is irrelevant to the core theoretical problem.
No scanner or outside program can flag malicious code with perfect accuracy because 'maliciousness' isn't an intrinsic property of a string of bytes or instructions — the same code (or string) can be benign in one program and harmful in another, depending on the context in which it executes and the intent behind it. This context-dependence is why the general problem of virus/malware detection is provably undecidable, not merely a matter of outdated signature databases. Firewalls not targeting malicious code, or not knowing the source language, are secondary/practical issues, not the fundamental reason 100% accuracy is unattainable.