Final answer:
ISAC organizations, NIST, and CERT are well-known threat intel sources that provide information, guidelines, and assistance on cybersecurity and the mitigation of cyber threats.
Step-by-step explanation:
The well-known threat intel sources among the options provided are a) ISAC organizations, b) NIST, and c) CERT. Information Sharing and Analysis Centers (ISACs) are member-driven organizations that gather and share information on cyber threats to critical infrastructure. The National Institute of Standards and Technology (NIST) provides guidelines and standards for cybersecurity, including frameworks for improving critical infrastructure cybersecurity. The Computer Emergency Response Team (CERT) is often part of a governmental or other institution and provides advice and assistance on cybersecurity, including alerts and mitigation of cyber threats.
Reddit, while a popular online platform where cybersecurity topics might be discussed, is not typically considered a professional or authoritative source of threat intelligence.