Final answer:
Nessus plug-ins are written in NASL, a scripting language specific to Nessus, not in any of the provided options like C, Python, Java, or Ruby.
Step-by-step explanation:
The Nessus plug-ins are written in the Nessus Attack Scripting Language (NASL), which is a scripting language specific to the Nessus vulnerability scanner. Although NASL has its own syntax and structure, it is not one of the programming languages listed in your multiple-choice options. Although NASL may incorporate components or functions written in C, the plug-ins themselves are specifically scripted in NASL for the purpose of Nessus security assessments.